导入

1.同包下可以直接使用

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2.不同包下要导入

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3.导入java包

package one;

import java.util.Date;

public class Four {

    public static void main(String[] args){
        Date date = new Date();
        System.out.println(date);
    }
}

或

package one;

import java.util.*;

public class Four {

    public static void main(String[] args){
        Date date = new Date();
        System.out.println(date);
    }
}

三种访问指示符

0.前言

作用域	当前类	同一package	子孙类	其他package
public	√	√	√	√
protected	√	√	√	×
friendly(default)	√	√	×	×
private	√	×	×	×

关于protected详细请去 第六章：类再生--->继承部分

1.public 可以在别的类中访问

package one;

public class Test {

    public int a = 100;
    public void sout(){
        System.out.println("测试包");
    }
}

package one;

public class Four {

    public static void main(String[] args){
        Test test = new Test();
        test.sout();
        System.out.println(test.a);
    }
}

测试包
100

Process finished with exit code 0

2.private 只能在自己的类中访问 但是可以提供public函数让别的类中也可以访问

package one;

public class Test {

    private int a = 100;

    public int getA() {
        return a;
    }
}

package one;

public class Four {

    public static void main(String[] args){
        Test test = new Test();
        System.out.println(test.getA());
    }
}

100

Process finished with exit code 0

3.protected 自己和子类可以使用 别的类访问类似于private 通过public 函数访问

1>没有重复变量

package one;

public class ChildATest extends Test {

    public int getSuperA(){
        return super.a;
    }

    public int MyA(){
        return a;
    }

    public void setA(int a){
        this.a = a;
        System.out.println(this.a);
        System.out.println(super.a);
    }
}

package one;

public class Four {

    public static void main(String[] args) {
        Test test = new Test();
        System.out.println(test.getA());

        ChildATest childATest = new ChildATest();
        System.out.println(childATest.getSuperA());
        System.out.println(childATest.MyA());
        childATest.setA(100000);
    }
}

100
100
100
100000
100000

Process finished with exit code 0

2>有重复变量

package one;

public class ChildATest extends Test {

    private int a = 10;
    public int getSuperA(){
        return super.a;
    }

    public int MyA(){
        return a;
    }

    public void setA(int a){
        this.a = a;
        System.out.println(this.a);
        System.out.println(super.a);
    }
}

package one;

public class Four {

    public static void main(String[] args) {
        Test test = new Test();
        System.out.println(test.getA());

        ChildATest childATest = new ChildATest();
        System.out.println(childATest.getSuperA());
        System.out.println(childATest.MyA());
        childATest.setA(100000);
    }
}

100
100
10
100000
100

Process finished with exit code 0

总结：
1.没有重复变量时 a, this.a, super.a 都指的是父亲的a
2.如果子类也定义了a, 那么只有super.a 指的是父亲的a， a, this.a 都是子类的a