in java throw,what,why?

August 9, 2025

This content originally appeared on DEV Community and was authored by rajeshwari rajeshwari

In Java, throw is a keyword used to explicitly throw an exception. Here’s a breakdown of the what, why, how, and when to use it:

What is `throw` in Java?

throw is used to manually trigger an exception during program execution.
It transfers control from the current block of code to the nearest catch block that can handle the exception.

throw new IllegalArgumentException("Invalid input");

Why use `throw`?

You use throw to:

Handle unexpected situations or errors in your code.
Validate input (e.g., null values, invalid arguments).
Provide custom error messages or throw custom exceptions.
Improve code robustness by catching problems early.

How to use `throw` in Java?

Syntax:

throw new ExceptionType("Error message");

Example:

public void setAge(int age) {
    if (age < 0) {
        throw new IllegalArgumentException("Age cannot be negative");
    }
    this.age = age;
}

Note: The object thrown must be of type Throwable (i.e., Exception or subclass).

Custom Exception Example:

class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

public void checkSomething(boolean condition) throws MyException {
    if (!condition) {
        throw new MyException("Condition failed");
    }
}

When to use `throw` in Java?

Use throw when:

You need to stop normal flow due to a problem.
Input is invalid or violates rules.
Something unexpected happens (e.g., file not found, null value).
You are implementing custom business logic exceptions.

Common Mistakes

Forgetting new when creating the exception:

  throw IllegalArgumentException("error"); // ❌ wrong
  throw new IllegalArgumentException("error"); // ✅ correct

Throwing a non-Throwable type:

  throw "error"; // ❌ wrong

Forgetting to declare or handle checked exceptions:

  public void riskyMethod() throws IOException {
      throw new IOException("Disk not found"); // must declare or handle
  }

Would you like an explanation of the difference between throw and throws too?