The users can define a subclass of this class to customize how to modify a method body. The overall architecture is similar to the strategy pattern.

If instrument() is called in CtMethod, the method body is scanned from the beginning to the end. Whenever an expression, such as a method call and a new expression (object creation), is found, edit() is called in ExprEdit. edit() can inspect and modify the given expression. The modification is reflected on the original method body. If edit() does nothing, the original method body is not changed.

The following code is an example:

CtMethod cm = ...;
cm.instrument(new ExprEditor() {
    public void edit(MethodCall m) throws CannotCompileException {
        if (m.getClassName().equals("Point")) {
            System.out.println(m.getMethodName() + " line: "
                               + m.getLineNumber());
    }
});

This code inspects all method calls appearing in the method represented by cm and it prints the names and the line numbers of the methods declared in class Point. This code does not modify the body of the method represented by cm. If the method body must be modified, call replace() in MethodCall.