Java常用类之System类的使用指南

import java.util.Arrays;

/**
 * System类演示
 */
public class SystemTest {
    public static void main(String[] args) {
        int[] src = {1, 2, 3};
        int[] dest = new int[3];
        /**
         * 参数1：源数组
         * 参数2：从源数组的那个位置开始拷贝
         * 参数3：目标数组
         * 参数4：把源数组的数据拷贝到目标数组的那个索引
         * 参数5：从源数组拷贝多少的数到目标数组
         */
        System.arraycopy(src, 0, dest, 0, 3);
        System.out.println(Arrays.toString(dest));
        // 返回当前时间距离1970年1月1日的毫秒数
        System.out.println(System.currentTimeMillis());
        // 调用垃圾回收机制
        System.gc();
        // 退出当前程序
        // 0：程序正常退出
        System.exit(0);
    }
}

package com.ibelifly.commonclass.system;

public class Test1 {
    public static void main(String[] args) {
        int[] arr={23,45,20,67,57,34,98,95};
        int[] dest=new int[8];
        System.arraycopy(arr,4,dest,4,4);
        for (int x:dest) {
            System.out.print(x+" ");
        }
    }
}

package com.ibelifly.commonclass.system;

public class Test2 {
    public static void main(String[] args) {
        System.out.println(System.currentTimeMillis()); //打印现在的时间
        long start=System.currentTimeMillis(); //该方法可用来计时
        for (int i = -99999999; i < 99999999; i++) {
            for (int j = -99999999; j < 99999999; j++) {
                int result=i+j;
            }
        }
        long end=System.currentTimeMillis();
        System.out.println("用时："+(end-start));
    }
}

package com.ibelifly.commonclass.system;

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

    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
    protected void finalize() throws Throwable {
        System.out.println("回收了"+name+" "+age);
    }
}

package com.ibelifly.commonclass.system;

public class Test3 {
    public static void main(String[] args) {
        new Student("小明",20);
        new Student("小红",28);
        new Student("小刚",22);
        System.gc();
    }
}

package com.ibelifly.commonclass.system;

​​​​​​​public class Test4 {
    public static void main(String[] args) {
        System.out.println("程序开始了");
        System.exit(0); //因为此处已经终止当前运行的Java虚拟机，故不会执行之后的代码
        System.out.println("程序结束了");
    }
}

// BigInteger
BigInteger bigInteger = new BigInteger("11111111111111111111111");
BigInteger bigInteger1 = new BigInteger("12345678910");
// +
BigInteger addRes = bigInteger.add(bigInteger1);
System.out.println(addRes);
// -
BigInteger subRes = bigInteger.subtract(bigInteger1);
System.out.println(subRes);
// *
BigInteger mulRes = bigInteger.multiply(bigInteger1);
System.out.println(mulRes);
// /
BigInteger divRes = bigInteger.divide(bigInteger1);
System.out.println(divRes);

// BigDecimal
BigDecimal bigDecimal = new BigDecimal("1.1111111111111111111999999999999");
BigDecimal bigDecimal1 = new BigDecimal("1.234567");
System.out.println(bigDecimal);
// +
BigDecimal daddRes = bigDecimal.add(bigDecimal1);
System.out.println(daddRes);
// -
BigDecimal dsubRes = bigDecimal.subtract(bigDecimal1);
System.out.println(dsubRes);
// *
BigDecimal dmulRes = bigDecimal.multiply(bigDecimal1);
System.out.println(dmulRes);
// /，注意：如果不指定精度，结果是死循环小数，会抛出一个异常
BigDecimal ddivRes = bigDecimal.divide(bigDecimal1,BigDecimal.ROUND_CEILING);
System.out.println(ddivRes);