Java—集合框架 contains()

• 如何個判斷集合中是否存在某個元素——contains()

　　1.List的contains(obj)方法

　　實際上，List調用contains(Object obj)方法時，會遍歷List中的每一個元素，然後再調用每個元素的equals()方法去跟contains()方法中的參數進行比較，如果有一個元素的equals()方法返回true則contains()方法返回true，否則所有equals()方法都不返回true，則ontains()方法則返回false。因此，重寫了Course類的equals()方法，否則，testListContains()方法的第二條輸出為false。

　　2.Set的Contains(obj)方法

　　當調用HashSet的contains(Object obj)方法時，其實是先調用每個元素的hashCode()方法來返回哈希碼，如果哈希碼的值相等的情況下再調用equals(obj)方法去判斷是否相等，只有在這兩個方法所返回的值都相等的情況下，才判定這個HashSet包含某個元素。因此，需重寫Course類的hashCode()方法和equals()方法。

　　以下代碼測試List和Set的contains()方法：

　　SetTest.java

package com.test.collection;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class SetTest {
    public List<Course> coursesToSelect;
    private Scanner console;
    public static Student student;
    public SetTest() {
        coursesToSelect = new ArrayList<Course>();
        console = new Scanner(System.in);
    }
    public void testAdd() {
        Course c1 = new Course("1", "數據結構");//創建課程對象的實例
        Course c2 = new Course("2", "C語言");
        Course c3 = new Course("3", "離散數學");
        Course c4 = new Course("4", "彙編語言");
        Course[] course = {c1, c2, c3, c4};
        coursesToSelect.addAll(Arrays.asList(course));
    }
    
    public void testForEach() {
        System.out.println("有以下課程可以選擇：(通過For Each)");
        for (Object obj : coursesToSelect) {
            Course c = (Course) obj;
            System.out.println("課程：" + c.id + ":" + c.name);
        }
    }

    /**
     * 測試List 的contains()方法
     */
    public void testListContains() {
        Course c = coursesToSelect.get(0);
        System.out.println("取得課程：" + c.name);
        System.out.println("課程【" + c.name + "】是否在備選課程中：" + coursesToSelect.contains(c));
        
        Course c2 = new Course(c.id, c.name);
        System.out.println("新創建課程：" + c2.name);
        System.out.println("課程【" + c2.name + "】是否在備選課程中：" + coursesToSelect.contains(c2));
        
        System.out.println("請輸入課程名稱：");
        String courseName = console.next();
        Course c3 = new Course();
        c3.name = courseName;
        System.out.println("課程【" + c3.name + "】是否在備選課程中：" + coursesToSelect.contains(c3));
        
    }
    
    /*
     * 創建學生並選課
     */
    public void createStudentAndSelectCourse() {
        student = new Student("1", "李雷");
        
        for (int i = 0; i < 3; i++) {
            System.out.println("請輸入課程編號：");
            String courseId = console.next();
            for(Course c : coursesToSelect ) {
                if (c.id.equals(courseId)) {
                    student.courses.add(c);
                }
            }
        }
    }
    
    public void testForEachForSet(Student student) {
        System.out.println("共選擇了" + student.courses.size() + "門課程！");
        for (Course c : student.courses) {
            System.out.println("選擇了課程：" + c.id + ":" + c.name);
        }
    }
    /**
     * 測試Set的contains()方法
     */
    public void testSetContains() {
        System.out.println("請輸入課程名稱：");
        String courseName = console.next();
        Course c = new Course();
        c.name = courseName;
        System.out.println("所選擇的課程中是否包含" + courseName + ":" + student.courses.contains(c));
        
    }
    
    public static void main(String[] args) {
        SetTest st = new SetTest();
        st.testAdd();
        st.testListContains();
        st.createStudentAndSelectCourse();
        st.testForEachForSet(SetTest.student);
        st.testSetContains();
    }
}

　　Course類：

package com.test.collection;

/**
 * 課程類
 * @author Administrator
 *
 */
public class Course {
    public String id;
    public String name;
    public Course(String id, String name){
        this.id = id;
        this.name = name;
    }
    public Course() {
        
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (!(obj instanceof Course))
            return false;
        Course other = (Course) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}