Java（静态）变量、（静态）代码块、构造方法的执行顺序
总结
1、父类静态变量和静态代码块（先声明的先执行）;
2、子类静态变量和静态代码块（先声明的先执行）;
3、父类的变量和代码块（先声明的先执行）;
4、父类的构造方法;
5、子类的变量和代码块（先声明的先执行）;
6、子类的构造方法。
注意事项
1、非静态代码块于构造方法之前执行,并且每次实例化对象之前都会先执行非静态代码块,静态代码块于非静态代码块之前执行;
2、在类第一次被调用时,加载该类,静态代码块只执行一次;
3、静态代码块只能调用静态变量,静态方法只能调用静态变量;
4、非静态代码块或非静态方法可以调用任何(静态+非静态)变量;
5、非静态代码块在实例化对象时,于构造方法之前执行。
参考代码 1
class HelloA {
public HelloA() {
System.out.println("HelloA");
}
{
System.out.println("I'm A class");
}
static {
System.out.println("static A");
}
}
public class HelloB extends HelloA {
public HelloB() {
System.out.println("HelloB");
}
{
System.out.println("I'm B class");
}
static {
System.out.println("static B");
}
public static void main(String[] args) {
new HelloB();
}
}
运行结果
```
static A
static B
I'm A class
HelloA
I'm B class
HelloB
```
static A
static B
I'm A class
HelloA
I'm B class
HelloB
```
参考代码 2
class A {
public A() {
System.out.println("HelloA");
}
{
System.out.println("I'm A class");
}
static {
System.out.println("static A");
}
}
public class B extends A {
public B() {
System.out.println("HelloB");
}
{
System.out.println("I'm B class");
}
static {
System.out.println("static B");
}
public static void main(String[] args) {
new B();
}
}
运行结果
```
static A
static B
I'm A class
HelloA
I'm B class
HelloB
```
static A
static B
I'm A class
HelloA
I'm B class
HelloB
```
参考代码 3
class A {
static {
System.out.print("1");
}
public A() {
System.out.print("2");
}
}
class B extends A {
static {
System.out.print("a");
}
public B() {
System.out.print("b");
}
}
public class Hello {
public static void main(String[] args) {
new B();
new B();
}
}
运行结果
```
1a2b2b
```
1a2b2b
```
参考代码 4
public class HelloA {
static {
System.out.println("static A");
}
{
System.out.println("I'm A class");
}
public HelloA() {
System.out.println("HelloA");
}
public HelloA(String s) {
System.out.println(s + "HelloA");
}
public static void main(String[] args) {
new HelloB();
}
}
class HelloB extends HelloA {
public HelloB() {
// 只能调用一次父类的构造方法，如果不写，Java 编译器会自动调用无参的构造方法。
// super();
super("parent");
System.out.println("HelloB");
}
{
System.out.println("I'm B class");
}
static {
System.out.println("static B");
}
}
运行结果
```
static A
static B
I'm A class
parentHelloA
I'm B class
HelloB
```
static A
static B
I'm A class
parentHelloA
I'm B class
HelloB
```
参考代码 5
public class People {
String name;
public People() {
System.out.println(1);
}
public People(String name) {
System.out.println(2);
this.name = name;
}
}
class Child extends People {
People father;
public Child() {
// super(); // 系统默认调用父类的无参构造方法
System.out.println(4);
}
public Child(String name) {
// super(); // 系统默认调用父类的无参构造方法
System.out.println(3);
this.name = name;
father = new People(name + ":F");
System.out.println(father.name);
}
public static void main(String[] args) {
new Child();
// new Child("mike");
}
}
运行结果
```
1
4
```
1
4
```
参考代码 6
public class People {
String name;
public People() {
System.out.println(1);
}
public People(String name) {
System.out.println(2);
this.name = name;
}
}
class Child extends People {
People father;
public Child() {
// super(); // 系统默认调用父类的无参构造方法
System.out.println(4);
}
public Child(String name) {
// super(); // 系统默认调用父类的无参构造方法
System.out.println(3);
this.name = name;
father = new People(name + ":F");
System.out.println(father.name);
}
public static void main(String[] args) {
// new Child();
new Child("mike");
}
}
运行结果
```
1
3
2
mike:F
```
1
3
2
mike:F
```
参考代码 7
public class ExA {
private static ExA a = new ExA();
static {
System.out.println("父类---静态代码块");
}
public ExA() {
System.out.println("父类---构造方法");
}
{
System.out.println("父类---非静态代码块");
}
public static void main(String[] args) {
new ExB();
}
}
class ExB extends ExA {
private static ExB b = new ExB();
static {
System.out.println("子类---静态代码块");
}
{
System.out.println("子类---非静态代码块");
}
public ExB() {
System.out.println("子类---构造方法");
}
}
运行结果
```
父类---非静态代码块
父类---构造方法
父类---静态代码块
父类---非静态代码块
父类---构造方法
子类---非静态代码块
子类---构造方法
子类---静态代码块
父类---非静态代码块
父类---构造方法
子类---非静态代码块
子类---构造方法
```
父类---非静态代码块
父类---构造方法
父类---静态代码块
父类---非静态代码块
父类---构造方法
子类---非静态代码块
子类---构造方法
子类---静态代码块
父类---非静态代码块
父类---构造方法
子类---非静态代码块
子类---构造方法
```
参考代码 8
class Foo {
public Foo(String word) {
System.out.println(word);
}
}
class Parent {
static Foo FOO = new Foo("Parent's static parameter");
Foo foo = new Foo("Parent's parameter");
static {
System.out.println("Parent's static code block");
}
{
System.out.println("Parent's code block");
}
public Parent() {
System.out.println("Parent.Parent()");
}
}
public class Child extends Parent {
static Foo FOO = new Foo("Child's static parameter");
Foo foo = new Foo("Child's parameter");
static {
System.out.println("Child's static code block");
}
{
System.out.println("Child's code block");
}
public Child() {
System.out.println("Child.Child()");
}
public static void main(String[] args) {
new Child();
}
}
运行结果
```
Parent's static parameter
Parent's static code block
Child's static parameter
Child's static code block
Parent's parameter
Parent's code block
Parent.Parent()
Child's parameter
Child's code block
Child.Child()
```
Parent's static parameter
Parent's static code block
Child's static parameter
Child's static code block
Parent's parameter
Parent's code block
Parent.Parent()
Child's parameter
Child's code block
Child.Child()
```
参考代码 9
public class GFG {
public GFG(int x) {
System.out.println("ONE argument constructor");
}
public GFG() {
System.out.println("No argument constructor");
}
static {
System.out.println("1st static init");
}
{
System.out.println("1st instance init");
}
{
System.out.println("2nd instance init");
}
static {
System.out.println("2nd static init");
}
public static void main(String[] args) {
new GFG();
new GFG(8);
}
}
运行结果
```
1st static init
2nd static init
1st instance init
2nd instance init
No argument constructor
1st instance init
2nd instance init
ONE argument constructor
```
1st static init
2nd static init
1st instance init
2nd instance init
No argument constructor
1st instance init
2nd instance init
ONE argument constructor
```
参考代码 10
class MyTest {
static {
initialize();
}
private static int sum;
public static int getSum() {
initialize();
return sum;
}
private static boolean initialized = false;
private static void initialize() {
if (!initialized) {
for (int i = 0; i < 100; i++)
sum += i;
initialized = true;
}
}
}
public class GFG {
public static void main(String[] args) {
System.out.println(MyTest.getSum());
}
}
运行结果
```
9900
```
9900
```
参考代码 11
class MyTest {
static {
initialize();
}
private static int sum = 0;
public static int getSum() {
initialize();
return sum;
}
private static boolean initialized = false;
private static void initialize() {
if (!initialized) {
for (int i = 0; i < 100; i++)
sum += i;
initialized = true;
}
}
}
public class GFG {
public static void main(String[] args) {
System.out.println(MyTest.getSum());
}
}
运行结果
```
4950
```
4950
```
参考代码 12
class MyTest {
static {
initialize();
}
private static int sum = 0;
public static int getSum() {
initialize();
return sum;
}
private static boolean initialized;
private static void initialize() {
if (!initialized) {
for (int i = 0; i < 100; i++)
sum += i;
initialized = true;
}
}
}
public class GFG {
public static void main(String[] args) {
System.out.println(MyTest.getSum());
}
}
运行结果
```
0
```
0
```
参考代码 13
class MyTest {
static {
initialize();
}
private static int sum;
public static int getSum() {
initialize();
return sum;
}
private static boolean initialized;
private static void initialize() {
if (!initialized) {
for (int i = 0; i < 100; i++)
sum += i;
initialized = true;
}
}
}
public class GFG {
public static void main(String[] args) {
System.out.println(MyTest.getSum());
}
}
运行结果
```
4950
```
4950
```
10-13 题解析
解析:[https://www.cnblogs.com/javaee6/p/3714716.html](https://www.cnblogs.com/javaee6/p/3714716.html)
参考资料