Day-7: InstanceOf and access modifiers: JAVA

🔍 What is instanceof?

The instanceof operator checks whether an object is an instance of a specific class, subclass, or interface. It returns a boolean result.

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📘 Summary of Key Concepts

1. Type Check: Checks if an object is of a given type.
2. Returns Boolean: Returns true if the object is of the specified type or a subtype.
3. Interfaces: Works with interfaces and superinterfaces.
4. Object Type Matters: Evaluates based on the actual object, not the declared variable type.
5. Superclass Match: Returns true if object is an instance of a superclass in the hierarchy.
6. Null Always False: instanceof always returns false when the object is null.
7. Compile-Time Safety: If the compiler knows the check is impossible (e.g., String instanceof Integer), it gives a compile error.
8. Safe Downcasting: Often used to check type before downcasting to avoid ClassCastException.
9. Pattern Matching: Allows type check and casting in one step: if (obj instanceof String s) { System.out.println(s.toUpperCase()); } 10.Short-Circuit Logic: Pattern variable can be used in same condition safely:

if (obj instanceof String s && s.length() > 5) {
    System.out.println(s);
}

TRICKY QUESTION

1. whats null instanceof null? Compile time error.instanceof requires the right-hand side to be a class or interface type.:
2. Can we check enum instanceof?

enum Color { RED, GREEN }
Color c = Color.RED;
System.out.println(c instanceof Color);

Yes,enums are classes and extend java.lang.Enum, so instanceof works with them.

1. whats int instanceof Integer? Compile-time error,instanceof cannot be used with primitives.

ACCESS MODIFIER:

Java provide 4 types of access modifiers, class/field/method defined as:

1. private: can only be accessed within same class not even in the subclasses. denoted by **private **keyword.
2. package-private: denoted by no keyword. this give visibility of entity through out same package.
3. protected : similar to package-private but its subclasses can also access protected entities. use **protected **keyword
4. public: can be accessed everywhere. use **public **keyword.