Java常用API

Object

tostring

package com.key.d13_packeg_04.API.Object;
public class Students{  //默认继承 extends Object
    private String name;
    private char sex;
    private int age;
    public Students() {
    }
    public Students(String name, char sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public char getSex() {
        return sex;
    }
    public void setSex(char sex) {
        this.sex = sex;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    /**
     * 重写tostring方法，以便返回内容信息
     * @return
     */
    @Override
    public String toString() {
        return "姓名：" + this.name;
    }
}

package com.key.d13_packeg_04.API.Object;
public class test1 {
    public static void main(String[] args) {
        /**
         *  掌握Object类中tostring方法的使用
         */
        Students stu = new Students("张三",'男',20);
        String s = stu.toString();
        System.out.println(s);
        //也可以直接打印stu，和调用tostring返回一致
        System.out.println(stu);
    }
}

equals

package com.key.d13_packeg_04.API.Object;
public class Students{  //默认继承 extends Object
    private String name;
    private char sex;
    private int age;
    public Students() {
    }
    public Students(String name, char sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public char getSex() {
        return sex;
    }
    public void setSex(char sex) {
        this.sex = sex;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    /**
     * 自己重写equals，自己定制相等规则
     * 两个对象内容一样，就认定是相等的
     * 被比较者 ==> obj
     * 比较者 ==> this，调用equals的对象，s1
     */
    @Override
    public boolean equals(Object obj) {
        //  1、判断对象obj是不是指定类型
        if (obj instanceof Students) {  //判断Obj是不是属于学生类，要用instanof
            /*
            if (this.name.equals(((Students) obj).name) && this.age == ((Students) obj).age && this.sex == ((Students) obj).sex) {
                return true;
            }
            else {
                return false;
            }
             */
            //可以把上面的if判断语句简化成一句代码
            return this.name.equals(((Students) obj).name) && this.age == ((Students) obj).age && this.sex == ((Students) obj).sex;
        }else {
            return false;
        }
    }
}

package com.key.d13_packeg_04.API.Object;
public class test2 {
    public static void main(String[] args) {
        Students s1 = new Students("张三",'男',20);
        Students s2 = new Students("张三",'男',20);
        //比较两个对象的地址是否相同
        System.out.println(s1.equals(s2));
        //equals和==效果相同
        System.out.println(s1 == s2);
    }
}

其实类中重写的equals方法也可以用IDEA自动生成的

Objects

Objects与Object是继承关系，Objects是从JDK1.7之后才有的，Objects的equals方法比交结果一致，但是更安全

StringBuilder

package com.key.d13_packeg_04.API.StringBuilder;
/**
 *  学习StringBuilder如何操作字符串
 */
public class demo1 {
    public static void main(String[] args) {
        StringBuilder str1 = new StringBuilder();
        str1.append("aaa");     //拼接字符串——append()
        str1.append(1);
        str1.append(false);
        str1.append(3.3);
        System.out.println(str1);
        /**
         *      另一种拼接方式——链式编程
         */
        StringBuilder str2 = new StringBuilder();
        str2.append("a").append("b").append("c");
        System.out.println(str2);
        //反转字符串：reverse()
        System.out.println(str2.reverse().append(123));
        //取字符串长度
        System.out.println(str2.length());
        /**
         *  StringBuilder只是拼接字符串的手段
         *  最终结果还是要恢复成String类型
         */
        StringBuilder str3 = new StringBuilder();
        str3.append(123).append("abc").append("[]");
        //check(str3);  这要会报错，因为str3为Stringbuilder对象，需要恢复成String类型
        //恢复成String
        String str4 = str3.toString();
        check(str4);
    }
    public static void check(String str){
        System.out.println(str);
    }
}

另一个实例，将数组转化为[1,2,3,4]的形式输出

package com.key.d13_packeg_04.API.StringBuilder;
import java.lang.reflect.Array;
public class shili_demo {
    public static void main(String[] args) {
        int number[] = {1,2,3,4};
        System.out.println(ToString(number));
    }
    public static String ToString(int[] number){
        //  开始将数组拼接为字符串
        StringBuilder str = new StringBuilder("[");
        //  将数组添加到字符串中
        for (int i = 0; i < number.length; i++) {
            str.append(number[i]).append(i == number.length-1 ? ']' : ',');
        }
        String str1 = str.toString();
        return str1;
    }
}

Math

System

BigDecimal