Java -2

常用API

System

可以获取当前时间，以此计算运行代码的时间
也可以控制代码的结束

//获取当前时间点-毫秒 1970 1-1 8:00
long num = System.currentTimeMillis();
System.out.println(num);//系统退出运行
System.exit(0);

Runtime

获取操作系统的线程大小
能从操作系统最多获取多少内存大小
已经获取的获取的内存大小-M 
剩下的内存大小
在固定时间关机与取消

//获取操作系统的线程大小
int nums = Runtime.getRuntime().availableProcessors();
System.out.println(nums);//能从操作系统最多获取多少内存大小
long l = Runtime.getRuntime().maxMemory();
System.out.println(l / 1024 / 1024);//已经获取的获取的内存大小
long L = Runtime.getRuntime().totalMemory();
System.out.println(L / 1024 / 1024);//剩下的内存大小
long LL = Runtime.getRuntime().freeMemory();
System.out.println(LL / 1024 / 1024);//在固定时间关机与取消
Runtime.getRuntime().exec("shutdown -s -t 3600");
Runtime.getRuntime().exec("shutdown -a");System.exit(0);

equals

import java.util.Objects;public class Equals {String name;int age;public Equals(String name, int age) {this.name = name;this.age = age;}public Equals() {}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;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Equals equals = (Equals) o;return age == equals.age && Objects.equals(name, equals.name);}@Overridepublic int hashCode() {return Objects.hash(name, age);}
}

public class BiEqual {public static void main(String[] args) {Equals a1 = new Equals("ax", 11);Equals a2 = new Equals("ax", 11);boolean w = a1.equals(a2);System.out.println(w);//这里的比较默认的是对象的地址，如果想要比较具体的参数值需重写String s = "ax";StringBuilder sb = new StringBuilder("ax");boolean equals = s.equals(sb.toString());boolean equals1 = s.equals(sb);boolean equals2 = sb.equals(s);System.out.println(equals);System.out.println(equals1);System.out.println(equals2);//String中的String中的equals方法是先判断是否为字符串，如果不是字符串那再继续比较其他属性//StringBuilder中没有重写特定的equals方法只能从obj中继承//obj中equals比较的是对象的地址}}

Objects下的equqls可以有非空判断

clone

浅克隆与深克隆（在于引用数据类型数组的更改变化）

对象拷贝：clone方法是protected只可以在子类中被调用，在这里需要重写

并且java类中要继承Cloneable接口

该接口没有抽象方法，是一个标记性接口

Cloneable实现当前类可以被克隆，没有实现当前类的对象不可以被克隆

浅克隆，浅拷贝：

javabean类

public class CloneStudent implements Cloneable {String name;int age;int[] arr;public CloneStudent(String name, int age, int[] arr) {this.name = name;this.age = age;this.arr = arr;}public CloneStudent() {}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;}public int[] getArr() {return arr;}public void setArr(int[] arr) {this.arr = arr;}public void getCloneArr() {for (int j : arr) {System.out.print(j + " ");}}@Overrideprotected Object clone() throws CloneNotSupportedException {return super.clone();}
}

实现类

public class CloneTest {public static void main(String[] args) throws CloneNotSupportedException {int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};CloneStudent cs = new CloneStudent("ax", 19, arr);CloneStudent css = (CloneStudent) cs.clone();cs.getArr()[0] = 100;System.out.println(cs.getAge() + " " + cs.getName());cs.getCloneArr();System.out.println();System.out.println(css.getAge() + " " + css.getName());css.getCloneArr();}
}

深克隆，深拷贝：

javabean（这里的clone方法需要重写需求）

public class CloneStudent implements Cloneable {String name;int age;int[] arr;public CloneStudent(String name, int age, int[] arr) {this.name = name;this.age = age;this.arr = arr;}public CloneStudent() {}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;}public int[] getArr() {return arr;}public void setArr(int[] arr) {this.arr = arr;}public void getCloneArr() {for (int j : arr) {System.out.print(j + " ");}}@Overrideprotected Object clone() throws CloneNotSupportedException {int[] a = this.arr;int[] a1 = new int[a.length];for (int i = 0; i < a.length; i++) {a1[i] = a[i];}CloneStudent clone = (CloneStudent) super.clone();clone.setArr(a1);return clone;}
}

测试类

public class CloneTest {public static void main(String[] args) throws CloneNotSupportedException {int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};CloneStudent cs = new CloneStudent("ax", 19, arr);CloneStudent css = (CloneStudent) cs.clone();cs.getArr()[0] = 100;System.out.println(cs.getAge() + " " + cs.getName());cs.getCloneArr();System.out.println();System.out.println(css.getAge() + " " + css.getName());css.getCloneArr();}
}

执行结果