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);
}
}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：
- 陈皓博客
- Java Tutor - Visualize Java code execution to learn Java online (also visualize Python, Java, JavaScript, TypeScript, Ruby, C, and C++ code)