java中常用工具类equalsdateMatharraylist，Collections多种排序map集合遍历

Posted 2022-11-14 weixin_ancenhw

1、如果使用equals方法判断两个数据是否相等，如果有一方为null，会出现java.lang.NullPointerException空指针异常。采用Objects.equals（）工具类，可以避免报错的出现

  public static void main(String[] args) 
        String a=null;
        String b="aa";
        //空指针异常
        System.out.println(a.equals(b));
        boolean equals = Objects.equals(a, b);
        System.out.println(equals);
    

2、Date 工具类使用说明，时间比较，时间转换毫秒计算

 public static void main(String[] args) 
        //获取当前时间
        getTime();
        //计算时间差值，可以将时间转换为毫秒相减，在转换为时间
        long currentTimeMillis = System.currentTimeMillis();
        //即 1970 年 1 月 1 日 00：00：00 GMT,CST:  CST (中国标准时间) 是 UTC+8 时区,比 UTC（协调世界时) 提前 8 个小时与 UTC 的时间偏差可写为 +08:00
        System.out.println(new Date(0l));
        long time = new Date(0l).getTime();
        System.out.println(time);
        System.out.println(currentTimeMillis);
        System.out.println(time - currentTimeMillis);

        //时间比较
        Date date1 = new Date();
        Date date2 = new Date(System.currentTimeMillis() + 1000 * 60);
        compareMethod(date1,date2);

    

    //比较两个时间的大小
    private static void compareMethod(Date date1, Date date2) 
        int compareTo = date1.compareTo(date2);
        if (compareTo==1)
            System.out.println("date1大于date2");
        else if(compareTo==0)
            System.out.println("date1等于date2");
        else if (compareTo==-1)
            System.out.println("date1小于date2");
        
    


    //获得当前时间毫秒
    private static void getTime() 
        Date date = new Date();
        System.out.println(date.getTime());
    

计算两个日期的天数

 public static void main(String[] args) throws ParseException 
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入日期格式:");
        String next = scanner.next();
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        Date yearDate = dateFormat.parse(next);
        Date currentDate = new Date();
        long diffDate = currentDate.getTime() - yearDate.getTime();
        long l = diffDate / 1000 / 60 / 60 / 24;
        System.out.println("相差日期:"+l);

    

利用系统自带的拷贝数组的工具类arrayCopy

//利用系统自带的拷贝数组
    public static void main(String[] args) throws ParseException, Exception 
        String arrTag[]="a","b","c","g";
        String arrRes[]="d","e","f","h";
        //拷贝数组
        
        arraycopy(arrTag,0,arrRes,2,2);
        System.out.println(Arrays.toString(arrRes));

    

3、Math方法的使用

 public static void main(String[] args) 

        Float a=13.5F;
        System.out.println(Math.round(a));   //环绕，四舍五入
        System.out.println(Math.floor(a));  //地板，向下取整
        System.out.println(Math.ceil(a));   //天花板，向上取整

    

打印结果

14
13.0
14.0

Process finished with exit code 0

4、集合添加多个数据，打乱集合拍寻

  public static void main(String[] args) 

        
        ArrayList<String> arrayList = new ArrayList<>();
        //向集合中添加多个字符
        Collections.addAll(arrayList, "a", "b", "c", "d");
        System.out.println(arrayList);
        //打乱集合的排序
        Collections.shuffle(arrayList);
        System.out.println(arrayList);
    

5、Collections多排序规则

 public static void main(String[] args) 


        ArrayList<String> arrayList = new ArrayList<>();
        //向集合中添加多个字符
        Collections.addAll(arrayList, "b", "a", "d", "c");
        Collections.sort(arrayList);
        System.out.println(arrayList);

        ArrayList<student> studentArrayList = new ArrayList<>();
        studentArrayList.add(new student("A", 23));
        studentArrayList.add(new student("B", 13));
        studentArrayList.add(new student("bC", 63));
        studentArrayList.add(new student("aC", 63));
        studentArrayList.add(new student("dC", 63));
        //按年龄排序，再按名称第一个字符排序
        Collections.sort(studentArrayList, new Comparator<student>() 
            @Override
            public int compare(student o1, student o2) 
                int i = o2.age - o1.age;
                if (i == 0) 
                    i = o1.name.charAt(0) - o2.name.charAt(0);
                
                return i;
            
        );
        System.out.println(studentArrayList);


    

    static class student implements Comparable<student> 
        private String name;
        private int age;

        public student() 
        

        @Override
        public String toString() 
            return "student" +
                    "name='" + name + '\\'' +
                    ", age=" + age +
                    '';
        

        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 int compareTo(student o) 
            return this.getAge() - o.getAge();
        
    

6、map集合遍历的几种方式

  public static void main(String[] args) 
          Map<String,String> map=new HashMap<>();
          map.put("A","value1");
          map.put("B","value2");
          map.put("C","value3");
          map.put("D","value4");
          map.put("E","value5");
//        System.out.println(map);
        Set<String> keySet = map.keySet();
        keySet.forEach(items->
            System.out.println(items+map.get(items));

        );

        Iterator<String> iterator = keySet.iterator();
        while (iterator.hasNext())
            String next = iterator.next();
            System.out.println(map.get(next));
        
        for (String s : keySet) 
            System.out.println(map.get(s));
        
        Set<Map.Entry<String, String>> entrySet = map.entrySet();
        for (Map.Entry<String, String> stringStringEntry : entrySet) 
            System.out.println(stringStringEntry.getKey());