java-throw-throws

Not every method can handle every error that occurs within it. Sometimes the right response is to notify the caller that something went wrong and let the caller decide how to handle it. Without a standard way to declare and propagate exceptions, error information would be lost.

The throw keyword manually creates and throws an exception. The throws keyword in a method signature declares that the method may throw one or more checked exceptions, letting callers know what to expect. throw is used inside the method body; throws is part of the method declaration.

When throw new SomeException() executes, the JVM unwinds the call stack looking for a matching catch block. The throws clause in the method signature is a compile-time contract: callers must either catch the declared exceptions or add them to their own throws clause.

• throw: Takes a Throwable instance (or subclass), never returns normally
• throws: Lists checked exception types a method may propagate
• RuntimeException: Does not need throws — unchecked by design
• Override rules: Subclass method cannot throw a broader checked exception than the parent override

• Built from: Java Exception Hierarchy — throw works with any Throwable subclass
• Builds into: Custom Exceptions — custom exceptions are thrown with throw
• Contrasts with: Try-Catch-Finally — throw propagates; try-catch handles
• Related: Java Polymorphism — method overriding has specific throws clause constraints

• Throws for RuntimeException: Legal but pointless — the compiler doesn’t enforce it
• Overriding and throws: Subclass cannot add new checked exception types to throws clause
• Exception chaining: Use throw new Cause(e) or initCause() to wrap exceptions
• throws Exception: Too broad — defeats the purpose of checked exceptions