常用API

 

 

System练习程序

package _3._195;

//system 的常用方法
public class _195 {
    public static void main(String[] args) {


        //System.out.println("开始");
        //System.exit(0); //终止Java虚拟机的类
        //System.out.println("，，");

    //及时类 计算现在距离1970年过了多少毫秒
        System.out.println("-------------");
        System.out.println(System.currentTimeMillis());
        System.out.println("-------------");
        System.out.println(System.currentTimeMillis() * 1.0 / 1000 / 60 / 60 / 24 / 365 + "年");
        System.out.println("-------------");


        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            System.out.println(i);
        }
        long end = System.currentTimeMillis();
        System.out.println("共耗时" + (end - start) + "毫秒");
    }
}

Object练习程序：toString（）

Student

package _3._196;

public class Student {
    private String name;
    private int age;
    public Student(){

    }

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

ObjectDemo

package _3._196;

public class ObjectDemo {
    public static void main(String[] args) {
        Student s=new Student();
        s.setName("马瑞麟");
        s.setAge(22);
        System.out.println(s);
        /*未重写Object类时，输出为   _3._196.Student@4eec7777
          重写后，输出为            Student{name='马瑞麟', age=22}
         */
        System.out.println(s.toString());

        /*
        给“s”溯源
        ---------------------------------------------
        public void println(Object x) {  //x=s
        String s = String.valueOf(x);
        if (getClass() == PrintStream.class) {
            // need to apply String.valueOf again since first invocation
            // might return null
            writeln(String.valueOf(s));
        } else {
            synchronized (this) {
                print(s);
                newLine();
                }
            }
        }
        ----------------------------------------------
         public static String valueOf(Object obj) {  //obj=x=s
        return (obj == null) ? "null" : obj.toString();
        }
        ---------------------------------------------
        public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
        }
        ---------------------------------------------
         */
    }
}

Object练习程序：equals（）

Student

package _3._197;

import java.util.Objects;

public class Student {
    private String name;
    private int age;
    public Student(){

    }

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }


}

ObjectDemo

package _3._197;

public class ObjectDemo {
    public static void main(String[] args) {
        Student s1=new Student();
        s1.setName("马瑞麟");
        s1.setAge(22);
        Student s2=new Student();
        s2.setName("马瑞麟");
        s2.setAge(22);

        //需求  比较两个对象的内容是否相同
        System.out.println(s1==s2);

        System.out.println(s1.equals(s2));//如果没重写  false;   重写后  true



    }
}