常用API：Math、System、Object、Arrays、基本类型包装类、日期类、BigDecimal

Math

开方：
java.lang.Math.sqrt(x)

java.lang.Math.sqrt(4) 的结果是2

package com.zjm.API;

public class MathDemo {
    public static void main(String[] args) {
        //public static int abs(int a),返回参数的绝对值
        System.out.println(Math.abs(88));
        System.out.println(Math.abs(-88));
        System.out.println("-------");

        //public static double ceil(double a),返回大于或者等于参数的最小double值，等于一个整数
        System.out.println(Math.ceil(12.34));
        System.out.println(Math.ceil(12.56));
        System.out.println("-------");

        //public static double floor(double a),返回小于或者等于参数的最大double值，等于一个整数
        System.out.println(Math.floor(12.34));
        System.out.println(Math.floor(12.56));
        System.out.println("-------");

        //public static int round(float a/double a),四舍五入返回最接近参数的int
        System.out.println(Math.round(12.34));
        System.out.println(Math.round(12.56));
        System.out.println("-------");

        //public static int max/min(int a, int b),返回两个数的最大最小值
        System.out.println(Math.max(66, 88));
        System.out.println(Math.min(66, 88));
        System.out.println("-------");

        //public static double pow(double a, double b)，返回a的b次幂
        System.out.println(Math.pow(2, 3));
        System.out.println("-------");

        // public static double random(),返回值为double的正值，[0.0,1.0]
        System.out.println(Math.random());
        System.out.println(Math.random()*100);


    }
}

System

package com.zjm.API;

public class SystemDemo {
    public static void main(String[] args) {
//        System.out.println("开始");
//        //public static void exit(int status),终止当前运行的Java虚拟机，非零表示异常终止
//        System.exit(0);
//        System.out.println("结束");

        //public static native long currentTimeMillis()，返回距离1970年至今的时间，毫秒为单位
        System.out.println(System.currentTimeMillis());
        System.out.println(System.currentTimeMillis()*1.0/1000/60/60/24/365 + "年");

        //计算程序运行时长
        long start = System.currentTimeMillis();
        for (int i = 0; i < 100; i++) {
            System.out.println(i);
        }
        long end = System.currentTimeMillis();
        System.out.println("共耗时"+(end-start)+"毫秒");
    }
}

Object

package com.zjm.API;

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 String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", 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;

        if (age != student.age) return false;
        return name != null ? name.equals(student.name) : student.name == null;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }
}

package com.zjm.API;

public class ObjectDemo {
    public static void main(String[] args) {
        Student s = new Student("许乐乐",19);
　　　　　//直接输出对象变量默认调用toString方法，格式为s.toString()
        System.out.println(s); //com.zjm.API.Student@776ec8df
        System.out.println(s.toString()); //实际开发中数据的地址一般没有意义，toString方法应该可以直观的表示对象，所以建议所有类在创建时重写Object的toString类



        //需求：比较两个类
        Student s1 = new Student("许乐乐",19);
        Student s2 = new Student("许乐乐",19);
        System.out.println(s1==s2);//false
        System.out.println(s1.equals(s2));//重写前为false，重写后为true
        //“==”号对比的是双方的内存地址,Object的equal方法就是“==”,要比较对象内的值是否相同的话，需要在类中重写equal方法
        /*
        public boolean equals(Object obj) {
            return (this == obj);
        }
         */
    }

}