A simple example where basic String concatenation in Java is really better than a StringBuilder.

Foo.java

public class Foo {

	static final String aConstant = "a constant";

	public void concatSimpleString() {
		String aString = "one" + aConstant + "two";
	}

	public void concatWithStringBuilder() {
		StringBuilder sb = new StringBuilder();
		sb.append("one");
		sb.append(aConstant);
		sb.append("two");
		sb.toString();
	}

	public static void main(String[] args) {
		Foo foo = new Foo();
		foo.concatSimpleString();
		foo.concatWithStringBuilder();
	}
}

Javap.out

Compiled from "Foo.java"
public class Foo {
  static final java.lang.String aConstant;

  public Foo();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public void concatSimpleString();
    Code:
       0: ldc           #2                  // String onea constanttwo
       2: astore_1      
       3: return        

  public void concatWithStringBuilder();
    Code:
       0: new           #3                  // class java/lang/StringBuilder
       3: dup           
       4: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
       7: astore_1      
       8: aload_1       
       9: ldc           #5                  // String one
      11: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      14: pop           
      15: aload_1       
      16: ldc           #7                  // String a constant
      18: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      21: pop           
      22: aload_1       
      23: ldc           #8                  // String two
      25: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      28: pop           
      29: aload_1       
      30: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      33: pop           
      34: return        

  public static void main(java.lang.String[]);
    Code:
       0: new           #10                 // class Foo
       3: dup           
       4: invokespecial #11                 // Method "<init>":()V
       7: astore_1      
       8: aload_1       
       9: invokevirtual #12                 // Method concatSimpleString:()V
      12: aload_1       
      13: invokevirtual #13                 // Method concatWithStringBuilder:()V
      16: return        
}