Java日常使用容易出错的几个地方（一）

package com.company;
import java.math.BigDecimal;
/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
package com.company;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        String s = null;
        //System.out.println(s.equals("abc"));  会报java.lang.NullPointerException的错误
        System.out.println("abc".equals(s));
    }
}

package com.company;

import java.util.Objects;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        String s = null;
        System.out.println(Objects.equals(s,"abc"));
        System.out.println(Objects.equals("abc",s));
        //System.out.println(s.equals("abc"));
        //System.out.println("abc".equals(s));
    }
}
//看下源码就知道它不需要考虑空指针问题
    /**
     * Returns {@code true} if the arguments are equal to each other
     * and {@code false} otherwise.
     * Consequently, if both arguments are {@code null}, {@code true}
     * is returned and if exactly one argument is {@code null}, {@code
     * false} is returned.  Otherwise, equality is determined by using
     * the {@link Object#equals equals} method of the first
     * argument.
     *
     * @param a an object
     * @param b an object to be compared with {@code a} for equality
     * @return {@code true} if the arguments are equal to each other
     * and {@code false} otherwise
     * @see Object#equals(Object)
     */
    public static boolean equals(Object a, Object b) {
        return (a == b) || (a != null && a.equals(b));
    }

package com.company;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        int i1 = 15;
        Integer i2 = 15;
        Integer i3 = new Integer(15);
        System.out.println(i1 == i2);//i1和i2都是引用编译时期存放在常量池中的地址  true
        System.out.println(i2 == i3);//i3是在堆中新new出来的 所以地址不等于常量池中的地址 false
        //不在-128~127范围内,Integer都会在堆上新创建一个对象，不会复用已有的对象
        int n1 = 130;
        int n2 = 130;
        Integer n3 = 130;
        Integer n4 = 130;
        System.out.println(n1 == n2);//true
        System.out.println(n3 == n4);// false 在-128~127之内equals和==的结果一样  在这个范围之外如果仅仅比较值是否一样需要用equals来比较   巨坑
    }
}

package com.company;

import java.math.BigDecimal;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal("0.9");
        BigDecimal b = new BigDecimal("0.99");
        //-1 表示 a 小于 b，0 表示 a 等于 b ， 1表示 a 大于 b
        System.out.println(a.compareTo(b));// -1
    }
}

package com.company;

import java.math.BigDecimal;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        BigDecimal m = new BigDecimal("3.1415926");
        BigDecimal n = m.setScale(4,BigDecimal.ROUND_HALF_UP);
        System.out.println(n);// 3.1416
    }
}

package com.company;

import java.math.BigDecimal;

/**
 * @author 晓果冻
 * @version 1.0
 * @date 2021/6/23 7:45
 */
public class Demo {
    public static void main(String[] args) {
        Double d1 = 3.145926d;
        BigDecimal b1 = new BigDecimal(d1);//这种方式会丢失精度
        System.out.println(b1);//3.145925999999999778111714476835913956165313720703125
        BigDecimal b2 = new BigDecimal(String.valueOf(d1));
        BigDecimal b3 = BigDecimal.valueOf(d1);//BigDecimal.valueOf(d1)内部执行了Double的toString。Double的toString会自行截断
        BigDecimal b4 = new BigDecimal(d1.toString());
        System.out.println(b4);////3.145926
        System.out.println(b2);//3.145926
        System.out.println(b3);//3.145926
    }
}