new Vs composition

i wants to ask about the difference between the use of the "new" kewword:
EX:

Code :

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Student s=new Student();

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and the composition, for example:

Code :

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public class Proffessor{
Student advisee;     //an attribute
//details omitted

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The keyword "new" actually tells the JVM to allocate memory and instantiate a new object. This object can then be used through your variable.

When you dont call new and just create a variable the JVM automatically assigns the value of null to that variable unless its a primitive. This means you have a variable to play with but if you try to invoke any methods on that variable without having a proper object assigned to it you will get a NullPointerException.

// Json

thanks alot