The .class files (compiled Java) should not be under version control. The 'deployment' directory is ignored by the Modeler so that it is not uploaded. This means that everyone compiles the Java code themselves and there should be not incompatibility between JDK versions.
Did the deployment directory accidentally get added to version control? You can test this by adding a Java action, deploying and committing. If the file changes include a new .class file for the Java action, the deployment directory is under version control.
To fix this, you need TortoiseSVN 1.6 (!). Right-click the deployment directory and choose TortoiseSVN > Delete and add to ignore list > deployment.
Hope this helps.