Common Java Runtime Exceptions and Solutions

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While working with Java, you'll likely come across various runtime exceptions that can be frustrating and time-consuming to resolve. Runtime exceptions occur during the execution of a program and are usually caused by logic errors in your code. To save you from pulling your hair out, we've compiled a list of common Java runtime exceptions, their causes, and solutions to help you fix them quickly.

NullPointerException

The infamous NullPointerException occurs when you try to access a method or a field of an object that is null. In other words, you're trying to use an object that hasn't been initialized yet.

Example

public class Main {
    public static void main(String[] args) {
        String myString = null;
        System.out.println(myString.length());
    }
}

Solution

To fix a NullPointerException, make sure to initialize your objects before using them or add a null-check to ensure you don't try to access methods or fields of a null object.

public class Main {
    public static void main(String[] args) {
        String myString = "Hello, World!";
        System.out.println(myString.length());
    }
}

ArrayIndexOutOfBoundsException

An ArrayIndexOutOfBoundsException occurs when you try to access an array element with an index that is either negative or greater than or equal to the array's length.

Example

public class Main {
    public static void main(String[] args) {
        int[] myArray = {1, 2, 3};
        System.out.println(myArray[3]);
    }
}

Solution

To fix an ArrayIndexOutOfBoundsException, ensure that you're using a valid index within the bounds of the array. You can use the .length property of the array to avoid going out of bounds.

public class Main {
    public static void main(String[] args) {
        int[] myArray = {1, 2, 3};
        System.out.println(myArray[2]);
    }
}

ClassCastException

A ClassCastException occurs when you try to cast an object of one class to a different class that it cannot be cast to. This usually happens when you're using inheritance or implementing interfaces.

Example

public class Main {
    public static void main(String[] args) {
        Object myString = "Hello, World!";
        Integer myInteger = (Integer) myString;
    }
}

Solution

To fix a ClassCastException, make sure you're casting objects to their correct types. You can use the instanceof operator to check if an object is an instance of a specific class before casting.

public class Main {
    public static void main(String[] args) {
        Object myString = "Hello, World!";
        if (myString instanceof Integer) {
            Integer myInteger = (Integer) myString;
        } else {
            System.out.println("Cannot cast to Integer");
        }
    }
}

FAQ