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Immutable Objects and Fields To say that an object is immutable means that its state can’t be changed once it has been instantiated. lang package (for example, Integer, Float, and Boolean), which are called wrapper classes because they “wrap” functionality around a primitive type. String instances are probably the most commonly used type of immutable object, even though it might appear on the surface that you’re able to modify them. println(myString); //... println(myString); Running this code segment will produce the following output: Hi Hello Hello there From this example, it may seem that the object instance referenced by myString was modified twice after it was initially created: once when it was assigned a new value of “Hello” and a second time when “there” is appended.

In particular, you should use anonymous inner classes as event handlers when there’s a limited amount of functionality in the event handling method(s). CHAPTER 2 ■ DESIGNING LIBRARIES, CLASSES, AND METHODS Class Design An important part of being a professional object-oriented programmer is the ability to create well-designed classes. Practice is an important ingredient in mastering this skill, but some simple principles can help you become more effective. Class design is largely a matter of assigning responsibility, where you identify the functions that must be implemented and assign each one to the class or classes best suited to perform that function.

This diagram illustrates the relationship between the StudentReport, TestScore, and Student classes. Notice that both StudentReport and Student depend upon TestScore. This code is poorly designed because it violates both of the guidelines outlined previously: • The classes are tightly coupled because of an unnecessary dependency (specifically StudentReport’s dependency upon TestScore). • StudentReport suffers from weak cohesion, because it performs two functions: printing a report and calculating each student’s average.