Instance Variable Initialization in Java: Declaration vs. Constructor – Which is Better?

Instance Variable Initialization: Declaration vs. Constructor

When initializing instance variables in Java, there are two common approaches:

1. Instance Variable Initialization on Declaration

   class A {
       B b = new B();
   }

2. Instance Variable Initialization in Constructor

   class A {
       B b;
   
       A() {
            b = new B();
       }
   }

Is There an Advantage to Either Approach?

While the Java compiler treats both approaches identically, certain considerations should be made:

Initialization Order:
Both approaches initialize instance variables in the order they are declared in the code.

Readability:
The first approach (initialization on declaration) is generally considered more readable.

Exception Handling:
In the second approach (initialization in constructor), exception handling is possible. If the initialization throws an exception, the object creation will fail.

Initialization Block:
In addition to the above approaches, there is an initialization block, which is also placed in the constructor by the compiler.

{
    a = new A();
}

Best Practices:

In general, it is recommended to initialize instance variables on declaration unless there is a specific need for exception handling or lazy initialization. When working with dependencies, it is better to avoid using the new operator directly and instead opt for dependency injection.

Example:

If you have an ExpensiveObject that is costly to initialize, you can use lazy initialization to improve performance:

ExpensiveObject o;

public ExpensiveObject getExpensiveObject() {
    if (o == null) {
        o = new ExpensiveObject();
    }
    return o;
}

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