让我们先来看两个类：Base和Derived类。注意其中的whenAmISet成员变量，和方法preProcess()。

情景1：（子类无构造方法）

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);    }}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

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);    }}

输出结果是： set when declared

情景2：（子类添加了构造方法）

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);    }}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

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);    }}

输出结果为：set in constructor

情景3：（赋值的细节）

public class Singleton {    private static Singleton mInstance = new Singleton();  // 位置1    public static int counter1;    public static int counter2 = 0;    private Singleton() {        counter1++;        counter2++;    }    public static Singleton getInstantce() {        return mInstance;    }    public static void main(String[] args) {        Singleton singleton = Singleton.getInstantce();        System.out.println("counter1: " + singleton.counter1);        System.out.println("counter2: " + singleton.counter2);    }}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

public class Singleton {    private static Singleton mInstance;    public static int counter1;    public static int counter2;    static {        mInstance = new Singleton();        counter2 = 0;    }    private Singleton() {        counter1++;        counter2++;    }    public static Singleton getInstantce() {        return mInstance;    }    public static void main(String[] args) {        Singleton singleton = Singleton.getInstantce();        System.out.println("counter1: " + singleton.counter1);        System.out.println("counter2: " + singleton.counter2);    }}

• 在Prepare阶段，mInstance、counter1、counter2的初始值为（null，0，0）；
• 执行至 mInstance = new Singleton()时，进行实例创建并调用构造方法，使counter1、counter2变量的值改变为（1，1）；
• 执行counter2 = 0时，counter2的值再次置为0，最终程序的输出结果为：counter1: 1   counter2: 0

同理，以下代码的最终输出结果为：counter1: 1  counter2: 1

public class Singleton {    public static int counter1;    public static int counter2 = 0;    private static Singleton mInstance = new Singleton(); // 位置2    private Singleton() {        counter1++;        counter2++;    }    public static Singleton getInstantce() {        return mInstance;    }    public static void main(String[] args) {        Singleton singleton = Singleton.getInstantce();        System.out.println("counter1: " + singleton.counter1);        System.out.println("counter2: " + singleton.counter2);    }}

原因分析：

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