NullPointerException in Java
NullPointerException is a RuntimeException . Runtime exceptions are critical and cannot be caught at compile time. They crash the program at run time if they are not handled properly. When a class is instantiated, its object is stored in computer memory. The NullPointerExceptions occur when you try to use a reference that points to no location in memory (null) as though it were referencing an object. These include:
- Calling the instance method of a null object.
- Accessing or modifying the field of a null object.
- Throwing null as if it were a Throwable value.
The first line declares a variable named str, but, it does not contain a primitive value. Instead it contains a pointer (because the type is String which is a reference type). Since you did not say as yet what to point to Java sets it to null . That means its pointing at nothing.
In the second line, the new keyword is used to instantiate (or create) an object of type String and the pointer variable str is assigned this object. You can now reference the object using the dereferencing operator . (a dot).
The above code is to find the length of the string. When you run the code, it will output the result as 4.
The NullPointerException occurs when you declare a variable but did not create an object. If you attempt to dereference str before creating the object you get a NullPointerException. Check the following code:
The first line declares a variable named str and set to null. In the second line, you try to find the length of the String without instantiate (or create) an object of type String using "new" keyword. When you compile the code, the compiler generate .class file without any errors. But, when you run the above code, you will get "Exception in thread "main" java.lang.NullPointerException" . Here this exception is raised because when you try to access members from a class using object reference that is initialized to null value.
How to fix Java NullPointerException?
You can avoid this NullPointerException by coding like this:
Note: In order to fix this NullPointerException you should always initialize your objects before you try to do anything with them.
- Java Interview Questions-Core Faq - 1
- Java Interview Questions-Core Faq - 2
- Important features of Java
- What is the difference between JDK and JRE?
- What gives Java its 'write once and run anywhere' nature?
- What is JVM and is it platform independent?
- What is Just-In-Time (JIT) compiler?
- What is the garbage collector in Java?
- Difference between Stack and Heap memory in Java
- How to set the maximum memory usage for JVM?
- What is numeric promotion?
- Why do we need Generic Types in Java?
- What does it mean to be static in Java?
- What are final variables in Java?
- How Do Annotations Work in Java?
- How do I use the ternary operator in Java?
- What is instanceof keyword in Java?
- How ClassLoader Works in Java?
- What are fail-safe and fail-fast Iterators in Java
- What are method references?
- "Cannot Find Symbol" compile error
- Difference between system.gc() and runtime.gc()
- How to convert TimeStamp to Date in Java?
- Does garbage collection guarantee that a program will not run out of memory?
- How setting an Object to null help Garbage Collection?
- How do objects become eligible for garbage collection?
- How to calculate date difference in Java
- Difference between Path and Classpath
- Is Java "pass-by-reference" or "pass-by-value"?
- Difference between static and nonstatic methods java
- Why Java does not support pointers?
- What is package in Java?
- What are wrapper classes?
- What is singleton class in Java?
- Difference between Local Variable, Instance Variable and a Class Variable in Java?
- Can a top level class be private or protected in java
- Are Polymorphism , Overloading and Overriding similar concepts?
- How to Use Locks in Java
- Why Multiple Inheritance is Not Supported in Java
- Why Java is not a pure Object Oriented language?
- Why can't a Java class be declared as static?
- Difference between Abstract class and Interface in Java
- Why do I need to override the equals and hashCode methods in Java?
- Why does Java not support operator overloading?
- What’s meant by anonymous class in Java?
- Static Vs Dynamic class loading in Java
- Why am I getting a NoClassDefFoundError in Java?
- How to generate random integers within a specific range in Java
- What's the meaning of System.out.println in Java?
- What is the purpose of Runtime and System class?
- What is finally block in Java?
- What is difference between final, finally and finalize?
- What is try-with-resources in java?
- What is a stacktrace?