String源碼分析

類結構

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence 

String類實現了Serializable可以被序列化

String類實現了Comparable可以進行比較

String類實現了CharSequence可以按下標進行相關操作

並且String類使用final進行修飾，不可以被繼承

屬性

//用來存儲字元串的每一個字元
private final char value[];

//hash值
private int hash; // Default to 0

//序列化版本號
private static final long serialVersionUID = -6849794470754667710L;

//從變數名大致可以看出和序列化有關，具體的不明白
private static final ObjectStreamField[] serialPersistentFields =
        new ObjectStreamField[0];

構造方法

//無參，直接使用空字元串賦值，hash為0
public String() {
        this.value = "".value;
    }

//使用已有字元串初始化
public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }

//使用char數組初始化，hash為0
public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

//使用字元數組，並指定偏移、字元個數初始化
public String(char value[], int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= value.length) {
                this.value = "".value;
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > value.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        this.value = Arrays.copyOfRange(value, offset, offset+count);
    }

//使用unicode編碼數組並指定偏移和數量進行初始化
public String(int[] codePoints, int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= codePoints.length) {
                this.value = "".value;
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        // Pass 1: Compute precise size of char[]	計算char數組大小
        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))//判斷編碼是不是BMP（Basic Mutilingual Plane）
                continue;
            else if (Character.isValidCodePoint(c))//驗證編碼是否在unicode編碼範圍內
                n++;
            else throw new IllegalArgumentException(Integer.toString(c));
        }

        // Pass 2: Allocate and fill in char[] 申明char數組並填入編碼對應char
        final char[] v = new char[n];

        for (int i = offset, j = 0; i < end; i++, j++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))//如果編碼是BMP直接一個字元就是接受
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);//轉換成兩個字元存儲
        }

        this.value = v;
    }

//使用ascii碼數組進行初始化
@Deprecated
    public String(byte ascii[], int hibyte, int offset, int count) {
        checkBounds(ascii, offset, count);
        char value[] = new char[count];

        if (hibyte == 0) {
            for (int i = count; i-- > 0;) {
                value[i] = (char)(ascii[i + offset] & 0xff);
            }
        } else {
            hibyte <<= 8;
            for (int i = count; i-- > 0;) {
                value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));
            }
        }
        this.value = value;
    }

@Deprecated
    public String(byte ascii[], int hibyte) {
        this(ascii, hibyte, 0, ascii.length);
    }

//使用位元組數組+字元集名初始化
public String(byte bytes[], int offset, int length, String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null)
            throw new NullPointerException("charsetName");
        checkBounds(bytes, offset, length);
        this.value = StringCoding.decode(charsetName, bytes, offset, length);
    }

//使用位元組數組+字元集名初始化
public String(byte bytes[], int offset, int length, Charset charset) {
        if (charset == null)
            throw new NullPointerException("charset");
        checkBounds(bytes, offset, length);
        this.value =  StringCoding.decode(charset, bytes, offset, length);
    }

//使用位元組數組+字元集名初始化
public String(byte bytes[], String charsetName)
            throws UnsupportedEncodingException {
        this(bytes, 0, bytes.length, charsetName);
    }
//使用位元組數組+字元集名初始化
public String(byte bytes[], Charset charset) {
        this(bytes, 0, bytes.length, charset);
    }

//使用位元組數組初始化
public String(byte bytes[], int offset, int length) {
        checkBounds(bytes, offset, length);
        this.value = StringCoding.decode(bytes, offset, length);
    }

public String(byte bytes[]) {
        this(bytes, 0, bytes.length);
    }

//使用StringBuffer初始化
public String(StringBuffer buffer) {
        synchronized(buffer) {
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }

//使用StringBuilder初始化
public String(StringBuilder builder) {
        this.value = Arrays.copyOf(builder.getValue(), builder.length());
    }

方法

靜態方法

join(CharSequence，CharSequence...）使用分隔符拼接字元串

public static String join(CharSequence delimiter, CharSequence... elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        // Number of elements not likely worth Arrays.stream overhead.
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

join(CharSequence，Iterable<? extends CharSequence>）使用分隔符拼接字元串

public static String join(CharSequence delimiter,
            Iterable<? extends CharSequence> elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

format(String，Object...）使用字元串格式指定參數進行格式化生成字元串

public static String format(String format, Object... args) {
        return new Formatter().format(format, args).toString();
    }

format(Local，String，Object...）根據環境使用字元串格式指定參數進行格式化生成字元串

public static String format(Locale l, String format, Object... args) {
        return new Formatter(l).format(format, args).toString();
    }

valueOf(Object）對象轉換成字元串，如果對象為null轉為為字元串“null”

public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

valueOf(char[]）char數組轉換成字元串

public static String valueOf(char data[]) {
        return new String(data);
    }

valueOf（xxx）xxx數據類型轉換為字元串

public static String valueOf(boolean b) {
        return b ? "true" : "false";
    }

public static String valueOf(char c) {
        char data[] = {c};
        return new String(data, true);
    }

public static String valueOf(int i) {
        return Integer.toString(i);
    }

public static String valueOf(long l) {
        return Long.toString(l);
    }

public static String valueOf(float f) {
        return Float.toString(f);
    }

 public static String valueOf(double d) {
        return Double.toString(d);
    }

valueOf(char[],int,int）char數組按照偏移個指定字元格式轉換為字元串

public static String valueOf(char data[], int offset, int count) {
        return new String(data, offset, count);
    }

copyValueOf(char，int，int）使用指定字元數組根據偏移和字元個數拷貝一個新字元串，同valueOf

public static String copyValueOf(char data[], int offset, int count) {
        return new String(data, offset, count);
    }

copyValueOf(char[])使用指定字元數組拷貝新字元串

public static String copyValueOf(char data[]) {
        return new String(data);
    }

成員方法

char charAt(int index)獲取指定下標的字元

public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }

void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin)把當前字元串的char數組的指定範圍拷貝到目標char數組的指定位置

public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
        if (srcBegin < 0) {
            throw new StringIndexOutOfBoundsException(srcBegin);
        }
        if (srcEnd > value.length) {
            throw new StringIndexOutOfBoundsException(srcEnd);
        }
        if (srcBegin > srcEnd) {
            throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
        }
  //System.arraycopy(Object src,  int  srcPos,Object dest, int destPos,int length)
  //src：要拷貝的源數組
  //srcPos：源數組拷貝的起始位置
  //dest：目標數組
  //destPost：拷貝到目標數組的起始位置
  //length：要拷貝元素的個數
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }

byte[] getBytes(String charsetName)根據字元集獲取字元串的編碼後的位元組數組

public byte[] getBytes(String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null) throw new NullPointerException();
        return StringCoding.encode(charsetName, value, 0, value.length);
    }

boolean equals(Object anObject)方法比較兩個字元串，重寫的Object方法

public boolean equals(Object anObject) {
        if (this == anObject) {//地址相等兩對象equals為true
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {//判斷兩字元串的字元個數
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])//有一個字元不相等最直接為false
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

contentEquals(CharSequence cs)判斷當前String與其他字元序列是否相等，與equals不同的是，equals只有當兩個對象都是String時equals才為true，contentEquals可以用來同其他StringBuffer、StringBuilder和其他字元序列進行比較

public boolean contentEquals(CharSequence cs) {
        // Argument is a StringBuffer, StringBuilder
        if (cs instanceof AbstractStringBuilder) {
            if (cs instanceof StringBuffer) {
                synchronized(cs) {//如果是StringBuffer那麼進行上鎖操作
                   return nonSyncContentEquals((AbstractStringBuilder)cs);
                }
            } else {//StringBuilder不上鎖
                return nonSyncContentEquals((AbstractStringBuilder)cs);
            }
        }
        // Argument is a String
        if (cs instanceof String) {
            return equals(cs);
        }
        // Argument is a generic CharSequence
        char v1[] = value;
        int n = v1.length;
        if (n != cs.length()) {
            return false;
        }
        for (int i = 0; i < n; i++) {//其他字元序列，一個一個字元進行比較
            if (v1[i] != cs.charAt(i)) {
                return false;
            }
        }
        return true;
    }

equalsIgnoreCase(String anotherString)兩個字元串忽略大小寫進行比較是否相等

public boolean equalsIgnoreCase(String anotherString) {
        return (this == anotherString) ? true
                : (anotherString != null)//不為空
                && (anotherString.value.length == value.length)//字元個數相等
                && regionMatches(true, 0, anotherString, 0, value.length);//忽略大小寫比較
    }

public boolean regionMatches(boolean ignoreCase, int toffset,
            String other, int ooffset, int len) {
        char ta[] = value;
        int to = toffset;
        char pa[] = other.value;
        int po = ooffset;
        // Note: toffset, ooffset, or len might be near -1>>>1.
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)value.length - len)
                || (ooffset > (long)other.value.length - len)) {
            return false;
        }
        while (len-- > 0) {
            char c1 = ta[to++];
            char c2 = pa[po++];
            if (c1 == c2) {
                continue;
            }
            if (ignoreCase) {
                // If characters don't match but case may be ignored,
                // try converting both characters to uppercase.
                // If the results match, then the comparison scan should
                // continue.
              //把兩個字元轉換成大寫的
                char u1 = Character.toUpperCase(c1);
                char u2 = Character.toUpperCase(c2);
                if (u1 == u2) {
                    continue;
                }
                // Unfortunately, conversion to uppercase does not work properly
                // for the Georgian alphabet, which has strange rules about case
                // conversion.  So we need to make one last check before
                // exiting.
              //轉換成大寫的不相等，在轉換成小寫的判斷
                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
                    continue;
                }
            }
            return false;
        }
        return true;
    }

int compareTo(String anotherString)進行字元串的比較

public int compareTo(String anotherString) {
        int len1 = value.length;
        int len2 = anotherString.value.length;
        int lim = Math.min(len1, len2);
        char v1[] = value;
        char v2[] = anotherString.value;

        int k = 0;
        while (k < lim) {
            char c1 = v1[k];
            char c2 = v2[k];
            if (c1 != c2) {//如果當前字元串的字元比參數的大返回正數，否則返回負數
                return c1 - c2;
            }
            k++;
        }
  //如果兩個字元串，長度小的字元串與長度大的前部分每個字元都相等，如果兩字元串長度相等返回0，當前字元串長度大於參數字元串返回整數，當前字元串長度小於參數字元串返回負數
        return len1 - len2;
    }

compareToIgnoreCase(String str)字元串忽略大小寫進行比較

public int compareToIgnoreCase(String str) {
        return CASE_INSENSITIVE_ORDER.compare(this, str);
    }

public int compare(String s1, String s2) {
            int n1 = s1.length();
            int n2 = s2.length();
            int min = Math.min(n1, n2);
            for (int i = 0; i < min; i++) {
                char c1 = s1.charAt(i);
                char c2 = s2.charAt(i);
                if (c1 != c2) {
                    c1 = Character.toUpperCase(c1);
                    c2 = Character.toUpperCase(c2);
                    if (c1 != c2) {
                        c1 = Character.toLowerCase(c1);
                        c2 = Character.toLowerCase(c2);
                        if (c1 != c2) {
                            //如果兩字元不相等，最後是轉換成小寫的進行比較
                            return c1 - c2;
                        }
                    }
                }
            }
            return n1 - n2;
        }

startsWith(String prefix)判斷字元串是否以指定字元串開頭

public boolean startsWith(String prefix) {
        return startsWith(prefix, 0);
    }

public boolean startsWith(String prefix, int toffset) {
        char ta[] = value;
        int to = toffset;
        char pa[] = prefix.value;
        int po = 0;
        int pc = prefix.value.length;
        // Note: toffset might be near -1>>>1.
        if ((toffset < 0) || (toffset > value.length - pc)) {
            return false;
        }
        while (--pc >= 0) {//迴圈給定首碼字元串長度
            if (ta[to++] != pa[po++]) {//首碼字元串字元和當前字元串字元比較
                return false;
            }
        }
        return true;
    }

boolean endsWith(String suffix)判斷字元串是否以指定字元串結尾

public boolean endsWith(String suffix) {
        return startsWith(suffix, value.length - suffix.value.length);
    }

int hashCode()獲取字元串的hashCode

public int hashCode() {
  //預設字元串hash為0，如果是用另一個字元串就等於另一個字元串的hash
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {//一個一個字元的變數
              //前面字元的hash*31+當前字元的ascii碼
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

int indexOf(int ch)根據unicode編碼獲取下標

public int indexOf(int ch) {
        return indexOf(ch, 0);
    }

public int indexOf(int ch, int fromIndex) {
        final int max = value.length;
        if (fromIndex < 0) {
            fromIndex = 0;
        } else if (fromIndex >= max) {//如果查找的起始位置超過了數組下標
            // Note: fromIndex might be near -1>>>1.
            return -1;
        }

        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
          //編碼是一個基本多語言編碼
            // handle most cases here (ch is a BMP code point or a
            // negative value (invalid code point))
            final char[] value = this.value;
            for (int i = fromIndex; i < max; i++) {
                if (value[i] == ch) {
                    return i;
                }
            }
            return -1;
        } else {
          //獲取需要使用兩個char存儲的編碼的下標
            return indexOfSupplementary(ch, fromIndex);
        }
    }

private int indexOfSupplementary(int ch, int fromIndex) {
        if (Character.isValidCodePoint(ch)) {//是一個合法的unicode編碼
            final char[] value = this.value;
            final char hi = Character.highSurrogate(ch);
            final char lo = Character.lowSurrogate(ch);
            final int max = value.length - 1;
            for (int i = fromIndex; i < max; i++) {
                if (value[i] == hi && value[i + 1] == lo) {
                    return i;
                }
            }
        }
        return -1;
    }

int lastIndexOf(int ch)獲取指定編碼從後往前搜索的第一個下標

public int lastIndexOf(int ch) {
        return lastIndexOf(ch, value.length - 1);
    }

public int lastIndexOf(int ch, int fromIndex) {
        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {//編碼是一個基本多語言unicode編碼，使用一個char存儲
            // handle most cases here (ch is a BMP code point or a
            // negative value (invalid code point))
            final char[] value = this.value;
            int i = Math.min(fromIndex, value.length - 1);
            for (; i >= 0; i--) {
                if (value[i] == ch) {
                    return i;
                }
            }
            return -1;
        } else {
          //編碼使用兩個char存儲
            return lastIndexOfSupplementary(ch, fromIndex);
        }
    }

private int lastIndexOfSupplementary(int ch, int fromIndex) {
        if (Character.isValidCodePoint(ch)) {
            final char[] value = this.value;
            char hi = Character.highSurrogate(ch);
            char lo = Character.lowSurrogate(ch);
            int i = Math.min(fromIndex, value.length - 2);
            for (; i >= 0; i--) {
                if (value[i] == hi && value[i + 1] == lo) {
                    return i;
                }
            }
        }
        return -1;
    }

int indexOf(String str)獲取指定字元串的的第一個字元在當前字元串的下標

public int indexOf(String str) {
        return indexOf(str, 0);
    }

public int indexOf(String str, int fromIndex) {
        return indexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }

static int indexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) {
        if (fromIndex >= sourceCount) {
            return (targetCount == 0 ? sourceCount : -1);
        }
        if (fromIndex < 0) {
            fromIndex = 0;
        }
        if (targetCount == 0) {
            return fromIndex;
        }

        char first = target[targetOffset];
        int max = sourceOffset + (sourceCount - targetCount);

        for (int i = sourceOffset + fromIndex; i <= max; i++) {
            /* Look for first character. */
            if (source[i] != first) {
                while (++i <= max && source[i] != first);
            }

            /* Found first character, now look at the rest of v2 */
            if (i <= max) {
                int j = i + 1;
              //計算終止下標
                int end = j + targetCount - 1;
                for (int k = targetOffset + 1; j < end && source[j]
                        == target[k]; j++, k++);

                if (j == end) {
                    /* Found whole string. */
                    return i - sourceOffset;
                }
            }
        }
        return -1;
    }

String substring(int beginIndex)獲取指定下標到最末下標的字元串

public String substring(int beginIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
  //計算長度
        int subLen = value.length - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
    }

String substring(int beginIndex, int endIndex)根據起始下標和結束下標獲取字元串，包含起始下標字元，不包含結束下標字元

 public String substring(int beginIndex, int endIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        if (endIndex > value.length) {
            throw new StringIndexOutOfBoundsException(endIndex);
        }
   //計算字元個數
        int subLen = endIndex - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return ((beginIndex == 0) && (endIndex == value.length)) ? this
                : new String(value, beginIndex, subLen);
    }

String concat(String str)把參數字元串拼接到當前字元串

public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
  //獲取拼接字元串的長度
        int len = value.length;
  //把原字元串的字元拷貝到一個大小為原字元串大小+參數字元串大小的新數組中
        char buf[] = Arrays.copyOf(value, len + otherLen);
  //把拼接字元串的字元拷貝到數組中
        str.getChars(buf, len);
        return new String(buf, true);
    }

String replace(char oldChar, char newChar)把指定字元替換為新字元

public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {
            int len = value.length;
            int i = -1;
            char[] val = value; /* avoid getfield opcode */

            while (++i < len) {
                if (val[i] == oldChar) {//找到需要替換字元的位置
                    break;
                }
            }
            if (i < len) {
                char buf[] = new char[len];
                for (int j = 0; j < i; j++) {
                    buf[j] = val[j];
                }
                while (i < len) {
                    char c = val[i];
                    buf[i] = (c == oldChar) ? newChar : c;//把原字元替換為新字元
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;
    }

boolean matches(String regex)判斷正則表達式是否比配當前字元串

public boolean matches(String regex) {
        return Pattern.matches(regex, this);
    }

boolean contains(CharSequence s)判斷當前字元串是否包含另一個字元序列

public boolean contains(CharSequence s) {
        return indexOf(s.toString()) > -1;
    }

String trim()去掉字元串前後的空格

public String trim() {
        int len = value.length;
        int st = 0;
        char[] val = value;    /* avoid getfield opcode */

  //找到字元串由前往後第一個不是空格的位置
        while ((st < len) && (val[st] <= ' ')) {
            st++;
        }
  //找到字元串由後往前第一個不是空格的位置
        while ((st < len) && (val[len - 1] <= ' ')) {
            len--;
        }
        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
    }

char[] toCharArray()把字元串轉換成字元數組

public char[] toCharArray() {
        // Cannot use Arrays.copyOf because of class initialization order issues
        char result[] = new char[value.length];
  //使用System.arraycopy方法拷貝
        System.arraycopy(value, 0, result, 0, value.length);
        return result;
    }

本地方法

native String intern();獲取字元串所指向的字元串常量池中對象的地址

@Test
	public void test8() {
		String s1="abc";
		String s2=new String("abc");
		System.out.println(s1==s2);//false
		System.out.println(s1==s2.intern());//true
	}

使用s1="abc"這種方式棧中變數s1直接指向字元串常量池中的常量“abc”，而s2=new String("abc")這種方式，棧中變數s2指向的是對中一個變數t，t指向字元串常量池中的“abc”，所以s1和s2指向的地址不相同

s2.intern()獲取的是字元串的常量池中的地址，也就是如果變數直接指向常量池，那麼就是變數的地址，如果變數指向堆，那麼會獲取堆所指向字元串常量池中的地址