Cases where java.lang.nullpointerexception is thrown

We often have this error in our programs. java.lang.NullPointerException. This bug is started when you try to use a null object that has no value in the following cases:

Call a null method instance.
Access or modify the instance of a null object.
The exception is intended to throw.
When calling the length of the null array.
When accessing null locations such as an array.
When synchronizing a null object or using it inside a synchronized block.

What is the means that determines the cause and stops the exception that terminated the execution of the program?

Objects are referenced by pointers in java

An object is a class instance or an array, references are pointers to these objects and a null reference is Refers to no object.
When you declare a variable, Java sets the x variable to 0 and a pointer is created that references that object. For example, in the following code we will declare a variable x:

int x;
x = 5;

In this example, the variable x is set to zero by default, and the second line, the value 5, is written to the memory location pointed to by x. Java forbids the programmer to manipulate memory boxes, it gives you access only to the object's methods and not the memory location.

Cases or NullPointerException is thrown

1) Call a method of a null

Point dot = null; 
dot.getX();

Exception in thread "main" java.lang.NullPointerException
 at Test.main(Test.java:27)

2) Access or modify the instance of a null object.

Accessing a null reference raises the exception NullPointerException:

Point dot = null; 
dot.getX();

Exception in thread "main" java.lang.NullPointerException
 at Test.main(Test.java:38)

3) When using throw null

throw verifies that the argument is non-null, and if it is zero, it rises NullPointerException.

IOException nullException = null; 
try {
 throw nullException; 
} catch (IOException e) {
 e.printStackTrace(); 
}

4) When calling the null array length

char[] ch = null; 
int length = ch.length;

5) When accessing null locations such as an array

An example is to access the boxes of a null.

char[] ch = null; 
char c = ch[0];

6) When synchronizing a null

You get NullPointerException If you are trying to synchronize an object null or put it inside a synchronized block.

Thread th=null; 
synchronized(th){
 System.out.print("this thread is synchronized"); 
}

What are the preventions

The best way to avoid this type of exception In case you didn't create the object yourself is to check if the object is zero. The code should be written like this:

void display(String phrase){
 //check if sentence is not null
 if(phrase!=null)
 System.out.print(phrase); 
}

In IDEs such as Eclipse or Netbeans, the number of the line that triggered the error is displayed. This is a good way to identify the type of error.
Variables can have a default value like the String in this code:

String string="";

Initialize the checkboxes after declaring an array.

//Declaration
Thread[] th=new Thread[3]; 
//Initialization
th[0]=new Thread(); 
th[1]=new Thread(); 
th[2]=new Thread();

Use Boolean variables, counters,... etc to optimize your code.