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Core Java

11 — Constructors & Encapsulation

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▶️ java pkg1core/core26ConstructorsDemo.java · java pkg1core/core10Encapsulation.java

Constructors initialize objects

java
1class Employee {2    final String name;3    final int id;4 5    Employee(String name, int id) {    // constructor6        this.name = name;7        this.id = id;8    }9}
  • Same name as class, no return type
  • If you write no constructor, compiler adds a no-arg default
  • Once you define any constructor, default is not generated

Constructor chaining

java
1Employee() {2    this("Unknown", 0);    // must be first statement3}4 5Employee(String name, int id) {6    this(name, id, "General");7}8 9Employee(String name, int id, String dept) {10    this.name = name;11    this.id = id;12}

this(...) calls another constructor in the same class.
super(...) calls parent constructor (in subclasses).

Encapsulation — hide internal state

java
1class BankAccount {2    private double balance;   // hidden3 4    public double getBalance() { return balance; }5 6    public void deposit(double amt) {7        if (amt <= 0) throw new IllegalArgumentException("amt > 0");8        balance += amt;9    }10}
Access Who can access
private Same class only
package-private Same package
protected Package + subclasses
public Everyone

💡 Benefit: Invariants are protected — callers can't set balance = -999.

Immutable objects

Make fields final, no setters, defensive copies for mutable components.

java
1record Point(int x, int y) {}   // compiler-generated immutable class

Next → 12 Inheritance & Polymorphism Related → 01 Core Java

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