Create simple Test.java
public class Test {
  public static void main(String[] args) {
  }
}

Have JDK 7 installed
try javac -target 1.6 Test.java
FAIL: javac: target release 1.6 conflicts with default source release 1.7

try javac -target 1.6 -source 1.6 Test.java
Success.

The same when you try to use -target 1.5 or -target 1.4.2