Here is my class, where i am concatenating two string.
String concatenate with null using + operator execute smoothly but throws NullPointerException with concate() method.
public class Test {
public static void main(String[] args) {
String str="abc";
String strNull=null;
System.out.println(strNull+str);
str.concat(strNull);
}
}
can anybody tell me the reason behind it ??
Case 1:
System.out.println(strNull+str); // will not give you exception
From the docs(String conversion)
If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).
Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.
Case 2:
str.concat(strNull); //NullPointer exception
If you see the source of concat(String str) it uses str.length(); so it would be like null.length() giving you a NullPointerException.
System.out.println(strNull+str) give an output of "nullabc"?
If the reference is null, it is converted to the string "null" will not throw nullpointer because toString() is not called on itnull + str is converted to "null"+"abc" according to the example above. so the output should be "nullabc"?
If you see in java.lang.String source, and use null as parameter. NPE is thrown in first line in length() method.
public String concat(String str) {
int otherLen = str.length();//This is where NullPointerException is thrown
if (otherLen == 0) {
return this;
}
getChars(0, count, buf, 0);
str.getChars(0, otherLen, buf, count);
return new String(0, count + otherLen, buf);
}
null before doing anything.null + str throw a NullPointerException?@Blip : 1) null + "ABC"
Straight forward answer to your question is + operator in java uses StringBuilder.append method to concat the strings.
And StringBuilder.append() method treats null object as "null" String. Hence null + "ABC" won't give Null Pointer exception but rather prints nullABC.
2) String.concat()
And yes, there is no null check before string.length() method in concat() function of String class. Hence it is throwing null pointer exception.
Hope this answers your question.
Because inside concat method in String class, length method is getting invoked on passed parameter , due to which NPE is thrown.
public String concat(String str) {
int otherLen = str.length();<------
if (otherLen == 0) {
return this;
}
int len = value.length;
char buf[] = Arrays.copyOf(value, len + otherLen);
str.getChars(buf, len);
return new String(buf, true);
}
null + str throw a NullPointerException?
If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).nullabc for the given code String strNull = null; System.out.println(strNull + "abc");Actual implementation of concate method is like this . this method need an object of type String as a parameter and if argument is null then it throws Null Pointer Exp.
public String concat(String paramString)
{
int i = paramString.length();
if (i == 0) {
return this;
}
int j = this.value.length;
char[] arrayOfChar = Arrays.copyOf(this.value, j + i);
paramString.getChars(arrayOfChar, j);
return new String(arrayOfChar, true);
}
String.concat() need an object of type String as a parameter.
There is no type which null is an instanceof. Refer JLS:
15.20.2 Type Comparison Operator instanceof
RelationalExpression: RelationalExpression instanceof ReferenceType At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.
concate() method creates new String() object under the hood.
where as + combines\ concatenates values by using toString() method. That's why when we use + it continents "somevalue".append(null).toString().
for reference see this question.
The String class holds an array (probably an ArrayList) of characters. When you call .concat() it goes through and adds every character from the second string to the first.
If the first String is null, there is nothing to add to, causing a NullPointer Exception. Try initializing Strings with "".
Initializing a null String with "" should work.
