獲取每月第一天最後一天 java

SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd"); 
        
        //獲取前月的第一天
        Calendar   cal_1=Calendar.getInstance();//獲取當前日期 
        cal_1.add(Calendar.MONTH, -1);
        cal_1.set(Calendar.DAY_OF_MONTH,1);//設置爲1號,當前日期既爲本月第一天 
        firstDay = format.format(cal_1.getTime());
        System.out.println("-----1------firstDay:"+firstDay);
        //獲取前月的最後一天
        Calendar cale = Calendar.getInstance();   
        cale.set(Calendar.DAY_OF_MONTH,0);//設置爲1號,當前日期既爲本月第一天 
        lastDay = format.format(cale.getTime());
        System.out.println("-----2------lastDay:"+lastDay);
        
        
        //獲取當前月第一天：
        Calendar c = Calendar.getInstance();    
        c.add(Calendar.MONTH, 0);
        c.set(Calendar.DAY_OF_MONTH,1);//設置爲1號,當前日期既爲本月第一天 
        String first = format.format(c.getTime());
        System.out.println("===============first:"+first);
        
        //獲取當前月最後一天
        Calendar ca = Calendar.getInstance();    
        ca.set(Calendar.DAY_OF_MONTH, ca.getActualMaximum(Calendar.DAY_OF_MONTH));  
        String last = format.format(ca.getTime());
        System.out.println("===============last:"+last);