Java常用类

Object类

• Object是所有类的父类 所有类都默认继承了Object类

hashcode

public class Demo {
    public static void main(String[] args) {
        String a = "张三";
        String b = "李四";

        System.out.println(a.hashCode()); //774889
        System.out.println(b.hashCode()); //842061

        System.out.println(a.hashCode() == b.hashCode()); //false
    }
}

equals

public class Demo1 {
    public static void main(String[] args) {
        String a = "likedx.com";
        String a1 = new String("likedx.com");

        System.out.println(a.equals(a1)); //true

        String b = "likedx"; 
        //这里用了字符串的拼接
        b += ".com";

        System.out.println(b.equals(a)); //true
    }
}

• equals和==有什么区别

  • equals用来比较字符串是否相同
  • ==用来比较字符串的内存地址

toString

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;
    }

    //这里可以用快捷键生成 alt+ins
    public String toString() {
        return "name = " + name + "\t" + "age = " + age;
    }
}

public class Demo2 {
    public static void main(String[] args) {
        Student s = new Student("张三",15);

        //这里调用的就是toString方法
        System.out.println(s); //name = 张三 age = 15
    }
}

getClass

public class Demo3 {
    public static void main(String[] args) {
        Demo3 d = new Demo3();

        System.out.println(d.getClass()); //class Demo3.Demo3
    }
}

Math类

public class Demo3 {
    public static void main(String[] args) {
        //abs 绝对值
        System.out.println("【绝对值】"+Math.abs(-10.3));//结果： 10.3
        System.out.println("【最大值】"+Math.max(10,30));//结果： 30
        System.out.println("【最小值】"+Math.min(10,30));//结果： 10
        System.out.println("【正弦值】"+Math.sin(3.56));//结果： -0.4063057021444168
        System.out.println("【对数值】"+Math.log(20));//结果： 2.995732273553991
    }
}

Random类

public class Demo3 {
    public static void main(String[] args) {
        Random random = new Random();

        //生成0-9之间的整数
        int num = random.nextInt(10);
        System.out.println(num);


        int math = (int) (Math.random() * 10);
        System.out.println(math);

        System.out.println("==========");

        //生成0-9之间的整数 用循环打印出来
        for (int i = 0; i < 10; i++) {
            int n = random.nextInt(10);
            System.out.println(n);
        }
    }
}

包装类

public class User {
    private int age;
    private Integer pic;

    public static void main(String[] args) {
        User  u = new User();
        
        System.out.println(u.age); //0
        System.out.println(u.pic); //null
    }
}

public class Demo4 {
    public static void main(String[] args) {
        int a = 10;

        // 自动装箱 int --- Integer
        Integer a1 = a;

        //自动拆箱 Integer --- int
        int i = a1.intValue();

        
        System.out.println(a); //10
        System.out.println(a1); //10
        System.out.println(i); //10
    }
}

类型互转

public class Demo5 {
    public static void main(String[] args) {
        //String 转换为int
        String s = "100";
        int i = Integer.parseInt(s);
        System.out.println("String转换int:" + "\t" + (i+1)); //String转换int:  101

        //int转换为String
        int i1 = 100;
        String ss = String.valueOf(i1);
        System.out.println("int转换为String" + "\t" + ss+1); //int转换为String   1001

        //int转换为Integer
        Integer integer = 100;
        System.out.println("int转换为Integer" + "\t" + integer); //int转换为Integer  100

        //Integer转换为int
        Integer integer1 = 100;
        int i2 = integer1.intValue();
        System.out.println("Integer转换为int" + "\t" + i2); //Integer转换为int   100

        //String转换为Integer
        String s1 = "100";
        Integer integer2 = Integer.valueOf(s1);
        System.out.println("String转换为Integer" + "\t" + integer2); //String转换为Integer   100

        //Inreger转换为String
        Integer integer3 = 100;
        String s2 = String.valueOf(integer3);
        System.out.println("Inreger转换为String" + "\t" + s2+1); //Inreger转换为String   1001
    }
}