java.lang.Object Methods Overview

## Why every object has these methods In Java, **`java.lang.Object`** sits at the very top of the class hierarchy. If you write `class Foo {}` with no `extends`, the compiler implicitly makes it `class Foo extends Object`. So *every* object — including arrays — inherits Object's methods. This guarantees a common minimum API that the JDK and frameworks can rely on (collections, debuggers, `System.out.println`, etc.). ## The inherited methods, one by one - **`public boolean equals(Object obj)`** — *logical equality*. The default implementation compares **references** (`this == obj`): two variables are equal only if they point to the very same object. Override it to define equality by content (e.g. two `Point(1,2)` instances being equal). - **`public int hashCode()`** — returns an `int` used to bucket the object in **hash-based collections** (`HashMap`, `HashSet`). The default derives a value from the object's identity. Rule: if two objects are `equals`, they **must** have the same `hashCode`, so you override the two together. - **`public String toString()`** — a textual representation. Default is `getClass().getName() + "@" + Integer.toHexString(hashCode())`, e.g. `com.acme.Point@1b6d3586`. Override to produce something readable; it is auto-invoked in string concatenation and `println`. - **`public final Class<?> getClass()`** — returns the **runtime** `Class` object describing the actual type. It is **`final`**, so you cannot override it — the JVM must report the true type for reflection and safety. - **`protected Object clone()`** — produces a *field-by-field (shallow)* copy. It is **`protected`** and only works if the class implements the **`Cloneable`** marker interface; otherwise it throws `CloneNotSupportedException`. Effective Java discourages it in favor of copy constructors/factories. - **`protected void finalize()`** — historically called by the garbage collector before reclaiming an object. **Deprecated since Java 9** (unpredictable, slow, can resurrect objects); replaced by try-with-resources and `Cleaner`. - **`public final void wait()` / `wait(long)` / `wait(long,int)`, `public final void notify()`, `public final void notifyAll()`** — the intrinsic-lock **wait/notify** thread-coordination protocol. They are **`final`** (the protocol is fixed) and must be called while holding the object's monitor (else `IllegalMonitorStateException`). ## Which are final / protected / deprecated, and why it matters | Method | Modifier | Can you override? | Note | |---|---|---|---| | `equals`, `hashCode`, `toString` | public | Yes | The three you normally override | | `getClass` | public **final** | No | Must report true runtime type | | `clone` | **protected** | Yes (often re-declared public) | Needs `Cloneable` | | `finalize` | protected, **deprecated** | Yes but don't | Use `Cleaner` instead | | `wait`/`notify`/`notifyAll` | public **final** | No | Monitor protocol is fixed | The modifiers encode design intent: `final` blocks overriding things the runtime depends on; `protected` limits casual external calls (clone, finalize); `deprecated` signals 'avoid'.