Why is there a call to "String.valueOf(Object)" in my string concatenation bytecode?

Question

I have the following two methods in a small microbenchmark app (running JDK 1.6): ``` public static String testStringBuilder3(String str1, String str2, String str3, String str4, String str5) { return new StringBuilder(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).toString(); } public static String testStringBuilder4(String str1, String str2, String str3, String str4, String str5) { return str1 + "-" + str2 + "-" + str3 + "-" + str4 + "-" + str5; } ``` I would expect the bytecode for these two methods to be identical. They are almost identical, with one small difference that I'd like to understand. Here is the bytecode for the first method (I can't get StackOverflow to format this correctly): ``` public static java.lang.String testStringBuilder3(java.lang.String, java.lang.String, java.lang.String, java.lang.String, java.lang.String); Code: 0: new #164; //class java/lang/StringBuilder 3: dup 4: aload_0 5: invokespecial #170; /…

Accepted Answer

From the JDK source:

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

It's making sure that first variable isn't null, otherwise it will throw a `NullPointerException`, as that is the expected behavior of `StringBuilder(String str)`

Example:

```
public class StringTest
{
  public static void main(String a[])
  {
    System.out.println(testStringBuilder3(null, null, null, null, null));
    System.out.println(testStringBuilder4(null, null, null, null, null));
  }
 public static String testStringBuilder3(String str1, String str2, String str3, String str4, String str5) {
    return str1 + "-" + str2 + "-" + str3 + "-" + str4 + "-" + str5;
  }
  public static String testStringBuilder4(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuilder(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).toString();
  }
}
```

Output:

```
null-null-null-null-null
Exception in thread "main" java.lang.NullPointerException
    at java.lang.StringBuilder.<init>(Unknown Source)
    at StringTest.testStringBuilder3(StringTest.java:15)
    at StringTest.main(StringTest.java:11)
```

Why is there a call to "String.valueOf(Object)" in my string concatenation bytecode?

Solution

Problem