Order of execution of Initialization blocks and Constructors in Java
Prerequisite : Static blocks, Initializer block, Constructor
In a Java program, operations can be performed on methods, constructors and initialization blocks.
Instance Initialization Blocks : IIB are used to initialize instance variables. IIBs are executed before constructors. They run each time when object of the class is created.
Initializer block : contains the code that is always executed whenever an instance is created. It is used to declare/initialize the common part of various constructors of a class.
Constructors : are used to initialize the object’s state. Like methods, a constructor also contains collection of statements(i.e. instructions) that are executed at time of Object creation.
Order of execution of Initialization blocks and constructor in Java
- Static initialization blocks will run whenever the class is loaded first time in JVM
- Initialization blocks run in the same order in which they appear in the program.
- Instance Initialization blocks are executed whenever the class is initialized and before constructors are invoked. They are typically placed above the constructors within braces.
1st static init 2nd static init 1st instance init 2nd instance init No argument constructor 1st instance init 2nd instance init ONE argument constructor
Note : If there are two or more static/initializer blocks then they are executed in the order in which they appear in the source code.
Now, predict the output of the following program-
- Loop in initialize function goes from 0 to 99. With that in mind, you might think that the program prints the sum of the numbers from 0 to 99. Thus sum is 99 × 100 / 2, or 4, 950. The program, however, thinks otherwise. It prints 9900, fully twice this value.
- To understand its behavior, let’s trace its execution.The GFG.main method invokes MyTest.getSum. Before the getSum method can be executed, the VM must initialize the class MyTest. Class initialization executes static initializers in the order they appear in the source.
- The MyTest class has two static initializers: the static block at the top of the class and the initialization of the static field initialized. The block appears first. It invokes the method initialize, which tests the field initialized. Because no value has been assigned to this field, it has the default boolean value of false.
- Similarly, sum has the default int value of 0. Therefore, the initialize method does what you’d expect, adding 4, 950 to sum and setting initialized to true. After the static block executes, the static initializer for the initialized field sets it back to false, completing the class initialization of MyTest. Unfortunately, sum now contains the 4950, but initialized contains false.
- The main method in the GFG class then invokes MyTest.getSum, which in turn invokes initialize method. Because the initialized flag is false, the initializeIf method enters its loop, which adds another 4, 950 to the value of sum, increasing its value to 9, 900. The getSum method returns this value, and the program prints it
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