[프로그래밍언어][Java(자바)] Null 처리 (Casting, Object.toString(), String.toString() 비교)

 

class Main {

    public static String valueOf2(Object obj) {
        return obj == null ? "null" : obj.toString();
    }

    public static String defaultValueOf(Object obj, String defaultValue) {
        return obj == null ? defaultValue : obj.toString();
    }

    public static String defaultEmptyValueOf(Object obj) {
        return obj == null ? "" : obj.toString();
    }

    public static String defaultEmptyValueOf2(Object obj, String defaultValue) {
        return String.valueOf(obj).replaceAll("null","");
    }
    
    public static void main(String[] args) {

        Object obj = null;
        String str1 = (String) obj;        // null
        String str2 = obj.toString();      // NullPointerException
        String str3 = String.valueOf(obj); // "null"
        
        String str4 = null;
        
        String str5 = "";
        
        System.out.println(str1);  // "null"
        System.out.println(str2);  // NullPointerException
        System.out.println(str3);  // "null"
        System.out.println(str4);  // "null"
        System.out.println(str5);  // ""
        
        System.out.println(str1.isEmpty());  // NullPointerException
        System.out.println(str1.length());   // NullPointerException
        
        System.out.println(str2.isEmpty());  // NullPointerException
        System.out.println(str2.length());   // NullPointerException
        
        System.out.println(str3.isEmpty());  // false
        System.out.println(str3.length());   // 4
        
        System.out.println(str4.isEmpty());  // NullPointerException
        System.out.println(str4.length());   // NullPointerException
        
        System.out.println(str5.isEmpty());  // true
        System.out.println(str5.length());   // 0
        
    }
}

 

1. Casting

 

2. Object.toString()

https://docs.oracle.com/javase/8/docs/api/

Java Platform SE 8

Modifier and Type	Method and Description
String	toString() Returns a string representation of the object.

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())

 

3. String.valueOf(Object obj)

https://docs.oracle.com/javase/8/docs/api/java/lang/String.html

String (Java Platform SE 8 )

Modifier and Type	Method and Description
static String	valueOf(Object obj) Returns the string representation of the Object argument.

Parameters:
    obj - an Object.

Returns:
    if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.