java 获取当月第一天和最后一天 获取前一个月第一天和最后一天

public static void main(String[] args) {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd"); 
        
        //获取前月的第一天
        Calendar   cal_1=Calendar.getInstance();//获取当前日期 
        cal_1.add(Calendar.MONTH, -1);
        cal_1.set(Calendar.DAY_OF_MONTH,1);//设置为1号,当前日期既为本月第一天 
        String firstDay = format.format(cal_1.getTime());
        System.out.println("-----1------firstDay:"+firstDay);
        
        //获取前月的最后一天
        Calendar cale = Calendar.getInstance();   
        cale.set(Calendar.DAY_OF_MONTH,0);//设置为1号,当前日期既为本月第一天 
        String lastDay = format.format(cale.getTime());
        System.out.println("-----2------lastDay:"+lastDay);
        
        
        //获取当前月第一天：
        Calendar c = Calendar.getInstance();    
        c.add(Calendar.MONTH, 0);
        c.set(Calendar.DAY_OF_MONTH,1);//设置为1号,当前日期既为本月第一天 
        String first = format.format(c.getTime());
        System.out.println("===============first:"+first);
        
        //获取当前月最后一天
        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);
        
    }

public static void main(String[] args) {

        Calendar cal = Calendar.getInstance(); 
        cal.setTime(new Date()); 
        cal.set(Calendar.DAY_OF_MONTH, 1); 
        System.out.println (new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime())); 
        cal.roll(Calendar.DAY_OF_MONTH, -1);  
        System.out.println (new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime()));
    }