java System類的一些靜態方法

package cn.sasa.demo2; public class SystemDemo { public static void main(String[] args) { func_arraycopy(); } static void func_1() { //currentTimeMillis() 獲取當前的毫秒數 返回值long
        long time1 = System.currentTimeMillis(); //System.out.println(time1);
        for(int i = 0; i<100000; i++) { System.out.println(i);//循環體裏有內容纔會執行循環
 } long time2 = System.currentTimeMillis(); //System.out.println(time2);
        System.out.println(time2 - time1); } static void func_2() { //exit() ---------終止程序
        while(true) { System.out.println("hi"); System.exit(0);//終止程序
 } //System.out.println("hello"); //由於前面有exit()，程序不會運行這一步，編譯報錯
 } static void func_gc() { //gc 垃圾回收 //當沒有引用再指向該對象時，JVM會自動回收堆中的對象，同時調用該回收對象的finalize()方法
        new Person("sa",12); new Person("sa",12); new Person("sa",12); new Person("sa",12); new Person("sa",12); new Person("sa",12); new Person("sa",12); new Person("sa",12); System.gc(); } static void func_3() { //System 類 getProperties 獲取系統參數
 System.out.println(System.getProperties()); System.out.println(System.getProperty("java.class.version")); } static void func_arraycopy() { //System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length) //src ----源數組 //srcPos ----源數組的起始索引 //dest ----目標數組 //destPos ----目標數組的起始索引 //length ----複製幾個
        Person[] src = new Person[10]; for(int i = 0; i<10; i++) { src[i] = new Person("sa",i); } Person[] dest = new Person[20]; for(int i = 0; i<10; i++) { dest[i] = new Person("user",i); } System.arraycopy(src, 5, dest, 2, 3); for(int i = 0; i<dest.length; i++) { String pstr = dest[i].toString(); System.out.println(pstr); } } }

package cn.sasa.demo2; public class Person { private String name; private int age; public void setName(String name) { this.name = name; } public String getName() { return this.name; } public void setAge(int age) { this.age = age; } public int getAge() { return this.age; } public Person(String name, int age) { this.name = name; this.age = age; } public void finalize() { System.out.println("測試gc ------ 垃圾回收了"); } public String toString() { return "name:"+this.name +" age:"+this.age; } }