static静态关键字

static：修饰成员变量，内存机制

static是什么、修饰成员变量的方法

package com.zixin.d1_static;

public class User {

    public static int onlineNumber = 161;

    private String name;
    private int age;

    public static void main(String[] args) {
        System.out.println(User.onlineNumber);

        User u =new User();
        u.name="张三";
        u.age=21;
        System.out.println(u.name);
        System.out.println(u.age);

        u.onlineNumber++;
        System.out.println(u.onlineNumber);

        User u2 =new User();
        u2.name="张三2";
        u2.age=22;
        System.out.println(u2.name);
        System.out.println(u2.age);

        u.onlineNumber++;
        System.out.println(u.onlineNumber);
        System.out.println(User.onlineNumber);
        System.out.println(onlineNumber);

    }
}

总结

static修饰成员变量的内存原理

static：修饰成员方法、内存机制

static修饰成员方法的基本用法

package com.zixin.d1_static;

public class Student {

    private String name;

    public static int getMax(int age1,int age2){
        return age1 > age2 ? age1 :age2;
    }

    public void study(){
        System.out.println(name +"在好好学习，天天向上");
    }

    public static void main(String[] args) {
        System.out.println(Student.getMax(10,3));

        System.out.println(getMax(10,32));

        Student s = new Student();
        s.name = "猪八戒";
        s.study();

        System.out.println(s.getMax(13,14));
    }
}

总结

static修饰成员方法的内存原理

static的注意事项

package com.zixin.d1_static;

public class Test3 {

    public static int onlineNumber = 10;
    public static void test2(){
        System.out.println("=====test2=====");
    }

    private String name;
    public void run(){
        System.out.println(name+"跑得快~");
    }
    public void go(){
        System.out.println(Test3.onlineNumber);
        System.out.println(onlineNumber);
        test2();
        System.out.println(name);
        run();
    }

    public static void test(){
        System.out.println(Test3.onlineNumber);
        System.out.println(onlineNumber);
        test2();
//        System.out.println(name);
//        run();

    }

    public static void main(String[] args) {

    }
}

static应用知识：工具类

package com.zixin.d2_static_util;

import java.util.Random;

public class zixinUtil {
    public static String createVerifyCode(int n){
        String code = "";
        String data = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        Random r =new Random();
        for(int i =0;i<n;i++){
            int index = r.nextInt(data.length());
            code +=data.charAt(index);
        }
        return code;

    }
}

package com.zixin.d2_static_util;

import java.util.Random;

public class Login {
    public static void main(String[] args) {
        System.out.println(zixinUtil.createVerifyCode(6));
    }
}

package com.zixin.d2_static_util;

import java.util.Random;

public class zixinUtil {

    private zixinUtil(){

    }
    public static String createVerifyCode(int n){
        String code = "";
        String data = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        Random r =new Random();
        for(int i =0;i<n;i++){
            int index = r.nextInt(data.length());
            code +=data.charAt(index);
        }
        return code;

    }
}

总结

1．工具类是什么，有什么好处?

• 内部都是一些静态方法，每个方法完成一个功能
• 一次编写，处处可用，提高代码的重用性。
  2.工具类有什么要求?
• 建议工具类的构造器私有化处理。
• 工具类不需要创建对象。

练习

package com.zixin.d2_static_util;

public class ArrayUtil {

    private ArrayUtil(){

    }

    public static String toString(int[] arr){

        if(arr == null){
            return null;
        }

        String result = "[";
        for (int i = 0; i < arr.length; i++) {
            result += (i == arr.length-1 ? arr[i] : arr[i] +",");
        }
        result +="]";
        return result;
    }
}

package com.zixin.d2_static_util;

public class TestDemo2 {
    public static void main(String[] args) {
        int [] arr = null;
        int [] arr1 = {};
        int [] arr2 ={12,23,44,88};

        System.out.println(ArrayUtil.toString(arr));
        System.out.println(ArrayUtil.toString(arr1));
        System.out.println(ArrayUtil.toString(arr2));
    }
}

static应用知识：代码块

代码块的分类、作用

package com.zixin.d3_static_code;

public class StaticDemo1 {

    public static String schoolName;

    static {
        System.out.println("------静态代码块被触发执行了------");
        schoolName = "黑马";
    }

    public static void main(String[] args) {
        System.out.println("------main方法执行------");
        System.out.println(schoolName);
    }
}

package com.zixin.d3_static_code;

public class StaticDemo2 {

    private String name;
    public StaticDemo2(){
        System.out.println("===无参构造器被触发执行===");

    }

    {
        name = "张三";
        System.out.println("===实例代码块被触发执行===");
    }

    public static void main(String[] args) {
        StaticDemo2 s1 = new StaticDemo2();
        System.out.println(s1.name);

        StaticDemo2 s2 = new StaticDemo2();
        System.out.println(s2.name);
    }
}

静态代码块的应用案例

package com.zixin.d3_static_code;

import java.util.ArrayList;

public class StatucTest3 {

    public static ArrayList<String> cards = new ArrayList<>();

    static {
        String[] sizes ={"3","4","5","6","7","8","9","10","J","Q","K","A","2"};
        String [] colors ={"♥","♠","♦","♣"};

        for (int i = 0; i < sizes.length; i++) {
            for (int j = 0; j < colors.length; j++) {
                String card = sizes[i]+colors[j];
                cards.add(card);
            }
        }
        cards.add("小🃏");
        cards.add("大🃏");

    }

    public static void main(String[] args) {
        System.out.println(cards);
    }
}

总结

1．静态代码块的作用是什么?

• 如果要在启动系统时对静态资源进行初始化，则建议使用静态代码块完成数据的初始化操作。

static应用知识：单例设计模式

设计模式、单例模式介绍、恶汉单例模式

package com.zixin.singleinstance;

public class SingleInstance {

    public static SingleInstance instance = new SingleInstance();
    private SingleInstance(){

    }
}

package com.zixin.singleinstance;

import com.sun.xml.internal.ws.api.model.wsdl.WSDLOutput;

public class Test1 {
    public static void main(String[] args) {
        SingleInstance s1 = SingleInstance.instance;
        SingleInstance s2 = SingleInstance.instance;
        System.out.println(s1==s2);
    }

}

总结

1．饿汉单例的实现步骤?
定义一个类，把构造器私有。
定义一个静态变量存储一个对象

懒汉单例模式

package com.zixin.singleinstance;

public class SingleInstance2 {
    private static SingleInstance2 instance;


    public static SingleInstance2 getInstance(){
        if(instance==null){
            instance = new SingleInstance2();
        }
        return instance;
    }

    private SingleInstance2(){

    }
}

package com.zixin.singleinstance;

public class Test2 {
    public static void main(String[] args) {
        SingleInstance2 s1 = SingleInstance2.getInstance();
        SingleInstance2 s2 = SingleInstance2.getInstance();
        System.out.println(s1==s2);
    }
}

总结

1．懒汉单例的实现步骤?

• 定义一个类，把构造器私有。
• 定义一个静态变量存储一个对象。
• 提供一个返回单例对象的方法