一文带你快速了解java中的static关键词

public class testClassA {
 public static int val = 1;
 static {
 System.err.println("11:" + val);
 val = 2;
 System.err.println("22:" + val);
 }
 public static int getVal() {
 return val;
 }
}
System.err.println(testClassA.getVal());

public class ClassA {
 public static String val = "1";
 
 public ClassA() {
 }
 public static class ClassB {//静态内部类
 public static int bVal = 2;
 }
 
 public class ClassC {
 public ClassC()
 {
 
 }
 
 private void name() {
 System.err.println("abc");
 }
 }
}

import static java.lang.System.*;

public class testClass {
 public static void main(String[] args) {
 err.println();//这里就不用system.err.println()了。
 }
}

public class ClassA {
 
 private static ClassA instance = null;
 
 public static synchronized ClassA getInstance() {//静态方法生成实例 懒汉模式 用到的时候再实例化
 if (instance == null) {
 instance = new ClassA();
 }
 return instance;
 }
 
 public void test() {
 System.err.println(1);
 }
}

public class ClassA {
 
 private static ClassA instance = new ClassA();// 饿汉模式，直接生成实例
 
 public static ClassA getInstance() {
 
 return instance;
 }
 
 public void test() {
 System.err.println(1);
 }
}

public class ClassA {
 
 private volatile static ClassA instance = null;
 
 public static ClassA getInstance() {
 if (instance == null) {
 synchronized (ClassA.class) {
 if (instance == null) {
  instance = new ClassA();
 }
 }
 }
 return instance;
 }
 
 public void test() {
 System.err.println(1);
 }
}

//静态内部类实现单例模式 兼容旧版jdk
public class ClassA {
 private ClassA() {
 
 }
 
 private static class InstanceClass {
 private static ClassA instance = new ClassA();
 }
 
 public static ClassA getInstance() {
 return InstanceClass.instance;
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 public void test() {
 System.err.println(1);
 }
}
ClassA.getInstance().test(); //使用方法