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Java is an object-oriented language. “Object-oriented” is a term that has become so commonly used as to have practically no concrete meaning. This chapter explains just what “object-oriented” means for Java. It covers:

• Classes and objects in Java
• Creating objects
• Garbage collection to free up unused objects
• The difference between class (or static) variables and instance variables, and the difference between class (or static) methods and instance methods
• Extending a class to create a subclass
• Overriding class methods and dynamic method lookup
• Abstract classes
• Interface types and their implementation by classes

If you are a C++ programmer, or have other object-oriented programming experience, many of the concepts in this list should be familiar to you. If you do not have object-oriented experience, don’t fear: This chapter assumes no knowledge of object-oriented concepts.

We saw in the last chapter that close analogies can be drawn between Java and C. Unfortunately for C++ programmers, the same is not true for Java and C++. Java uses object-oriented programming concepts that are familiar to C++ programmers, and it even borrows from C++ syntax in a number of places, but the analogies between Java and C++ are not nearly as strong as those between Java and C. [1] C++ programmers may have an easier time with this chapter than C programmers will, but they should still read it carefully and try not to form preconceptions about Java based on their knowledge of C++.

[1] As we’ll see, Java supports garbage collection and dynamic method lookup. This actually makes it a closer relative, beneath its layer of C-like syntax, to languages like Smalltalk than to C++.

 

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