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At the beginning of an object's life, the Java virtual machine (JVM) allocates enough memory on the heap to accommodate the object's instance variables. When that memory is first allocated, however, the data it contains is unpredictable. If the memory were used as is, the behavior of the object would also be unpredictable. To guard against such a scenario, Java makes certain that memory is initialized, at least to predictable default values, before it is used by any code.
Initialization is important because, historically, uninitialized data has been a common source of bugs. Bugs caused by uninitialized data occur regularly in C, for example, because it doesn't have built-in mechanisms to enforce proper initialization of data. C programmers must always remember to initialize data after they allocate it and before they use it. The Java language, by contrast, has built-in mechanisms that help you ensure proper initialization of the memory occupied by a newly-created object. With proper use of these mechanisms, you can prevent an object of your design from ever being created with an invalid initial state.
The Java language has three mechanisms dedicated to ensuring proper initialization of objects: instance initializers (also called instance initialization blocks), instance variable initializers, and constructors. (Instance initializers and instance variable initializers collectively are called "initializers.") All three mechanisms
result in Java code that is executed automatically when an object is created. When you allocate memory for a new object with
new operator or the
newInstance() method of class
Class, the Java virtual machine will insure that initialization code is run before you can use the newly-allocated memory. If you
design your classes such that initializers and constructors always produce a valid state for newly-created objects, there
will be no way for anyone to create and use an object that isn't properly initialized.
If you provide no explicit initialization to instance variables, they will be awarded predictable default initial values, which are based only on the type of the variable. Table 1 shows the default initial values for each of the variable types. (These are the default initial values for both instance and class variables. Local variables are not given default initial values. They must be initialized explicitly before they are used.)