JAVA構造時成員初始化的陷阱

讓我們先來看兩個類：Base和Derived類。註意其中的whenAmISet成員變數，和方法preProcess()。

情景1：（子類無構造方法）

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet = "set when declared";

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

當.java源代碼轉換成一個.class文件後，其轉換成類似下麵的等價代碼：

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet;

    {whenAmISet = "set when declared";}

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

輸出結果是： set when declared

情景2：（子類添加了構造方法）

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet = "set when declared";

    public Derived() {
        whenAmISet = "set in constructor";
    }

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

當.java源代碼轉換成一個.class文件後，其轉換成類似下麵的等價代碼：

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet;

    public Derived() {
        whenAmISet = "set when declared";
        whenAmISet = "set in constructor";
    }

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

輸出結果為：set in constructor

原因分析：

1. 陳皓博客
2. Java Tutor - Visualize Java code execution to learn Java online (also visualize Python, Java, JavaScript, TypeScript, Ruby, C, and C++ code)