Java Instance Variables: Declaration or Constructor Initialization – Which is Better?

Instance Variable Instantiation: Declaration vs. Constructor

In Java, there are two options for instantiating instance variables: during declaration or within the constructor. This article explores the advantages and differences associated with each approach.

Example 1: Variable Instantiation on Declaration

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

Example 2: Variable Instantiation in Constructor

class A {
    B b;

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

Compiler Behavior

Contrary to common belief, there is no actual difference between the two approaches. In both cases, the compiler generates instance variable initialization code and places it in the class constructor(s). This initialization code appears in the order of the field declarations.

Readable and Visual Clarity

The first variant (declaration-based instantiation) offers enhanced readability. By explicitly initializing variables during declaration, code becomes more straightforward and improves visual clarity.

Exception Handling Considerations

However, a potential drawback of the declaration-based approach is that exception handling cannot be used. If the initialization code throws an exception, the instance variable will remain uninitialized.

Additional Initialization Options

Java also provides an initialization block that, like instance variable initialization, is inserted into the constructor by the compiler. This block allows for complex initialization logic.

Lazy Initialization

In scenarios where field initialization is computationally expensive, it may be desirable to implement lazy initialization. This approach initializes the field only when it is first used:

ExpensiveObject o;

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

Dependency Management and Injection

Lastly, for optimal dependency management, it is advisable to avoid using the new operator within classes. Instead, dependency injection is preferred, allowing an external entity to instantiate and provide necessary dependencies.

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