java 中的 static 关键字

 1package com.journaldev.misc;
 2
 3public class StaticExample {
 4
 5    //static block
 6    static{
 7        //can be used to initialize resources when class is loaded
 8        System.out.println("StaticExample static block");
 9        //can access only static variables and methods
10        str="Test";
11        setCount(2);
12    }
13
14    //multiple static blocks in same class
15    static{
16        System.out.println("StaticExample static block2");
17    }
18
19    //static variable example
20    private static int count; //kept private to control its value through setter
21    public static String str;
22
23    public int getCount() {
24        return count;
25    }
26
27    //static method example
28    public static void setCount(int count) {
29        if(count > 0)
30        StaticExample.count = count;
31    }
32
33    //static util method
34    public static int addInts(int i, int...js){
35        int sum=i;
36        for(int x : js) sum+=x;
37        return sum;
38    }
39
40    //static class example - used for packaging convenience only
41    public static class MyStaticClass{
42        public int count;
43
44    }
45
46}

 1package com.journaldev.misc;
 2
 3public class TestStatic {
 4
 5    public static void main(String[] args) {
 6        StaticExample.setCount(5);
 7
 8        //non-private static variables can be accessed with class name
 9        StaticExample.str = "abc";
10        StaticExample se = new StaticExample();
11        System.out.println(se.getCount());
12        //class and instance static variables are same
13        System.out.println(StaticExample.str +" is same as "+se.str);
14        System.out.println(StaticExample.str == se.str);
15
16        //static nested classes are like normal top-level classes
17        StaticExample.MyStaticClass myStaticClass = new StaticExample.MyStaticClass();
18        myStaticClass.count=10;
19
20        StaticExample.MyStaticClass myStaticClass1 = new StaticExample.MyStaticClass();
21        myStaticClass1.count=20;
22
23        System.out.println(myStaticClass.count);
24        System.out.println(myStaticClass1.count);
25    }
26
27}

1StaticExample static block
2StaticExample static block2
35
4abc is same as abc
5true
610
720

 1package com.journaldev.test;
 2
 3public class A {
 4
 5    public static int MAX = 1000;
 6    
 7    public static void foo(){
 8    	System.out.println("foo static method");
 9    }
10}

 1package com.journaldev.test;
 2
 3import static com.journaldev.test.A.MAX;
 4import static com.journaldev.test.A.foo;
 5
 6public class B {
 7
 8    public static void main(String args[]){
 9    	System.out.println(MAX); //normally A.MAX
10    	foo(); // normally A.foo()
11    }
12}