java基本知识备注

java 代码
1.float精确位数运算

float a = 1234.5678f;    
java.text.DecimalFormat df =new java.text.DecimalFormat("#.00");     
String a=df.format(a);    
System.out.println(a);  //1234.56    
//写#的是有值就写值，没值就不写    
//写0的是有值就写值，没值就写0  

2. 强制性得到某个类的某个属性的类型

package test;    
   
public class TestAction {    
   
    private TestManager manager  = new TestManager();    
        
} 

package test;    
   
public class TestManager {    
   
} 

package test;    
   
import java.lang.reflect.Field;    
   
public class Test3 {    
    public static void main(String[] args){    
        try{    
                
            TestAction testAction = new TestAction();    
            Object object = testAction;    
            System.out.println(object.getClass());//class test.TestAction    
                
            Field field = object.getClass().getDeclaredField("manager");    
                
            boolean accessible = field.isAccessible();    
            field.setAccessible(true);    
        
            Object result = null;    
                
            result = field.get(object);    
                
            field.setAccessible(accessible);    
                
            System.out.println(result);    
        }catch(Exception e){    
            e.printStackTrace();    
        }    
    }    
}    

打印结果为：

class test.TestAction
test.TestManager@9cab16


3.arrayToString

/**  
 *  Convert an array of strings to one string.  
 *  Put the 'separator' string between each element.  
 * @param a  
 * @param separator  
 * @return  
 */  
public static String arrayToString(String[] a, String separator) {   
    StringBuffer result = new StringBuffer();   
    if(a==null){   
        return "";   
    }   
    if (a.length > 0) {   
        result.append(a[0]);   
        for (int i=1; i<a.length; i++) {   
            result.append(separator);   
            result.append(a[i]);   
        }   
    }   
    return result.toString();   
}