Java - Variables & Constants

Variables & Constants

Primitives vs References

Java has 8 primitive types stored by value, and reference types (objects/arrays) stored as references.

// primitives
byte b=1; short s=2; int i=3; long l=4L; float f=1.0f; double d=2.0; char c='A'; boolean ok=true;

// reference
String name = "Ada";           // reference to a String object
int[] nums = {1,2,3};          // reference to an array object

Initialization and Default Values

• Local variables must be initialized before use.
• Fields get default values (0, false, null) if not explicitly set.

public class Demo {
  private int count;        // defaults to 0
  void m() {
    int x;                  // local
    // System.out.println(x); // compile error: x not initialized
  }
}

Scope and Lifetime

• Scope is bounded by the nearest braces { }.
• Local variables exist until the block ends; fields live with the object instance.

Constants with final

final prevents reassignment. For objects, the reference can’t change, but the object may still be mutable.

final int PORT = 8080;        // cannot reassign
final StringBuilder sb = new StringBuilder();
sb.append("ok");              // allowed (mutating object)
// sb = new StringBuilder();  // not allowed (reassigning reference)

Type Inference (var)

From Java 10, var lets the compiler infer the type of local variables. It is still statically typed.

var list = new java.util.ArrayList(); // inferred as ArrayList
var total = 42;                                // inferred as int

Shadowing and Hiding

Inner scopes can define variables that shadow outer names. Favor distinct names to reduce confusion.

int value = 10;
{
  int value = 20; // shadows outer value
}

Immutability Guidelines

• Prefer final for fields and variables that should not change.
• Use immutable data structures where possible.

Try it

1. Declare a final constant and try to reassign it (should not compile).
2. Create a method that uses var for locals and replace with explicit types.
3. Demonstrate local vs field default values in a small class.