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An integer in Java is a memory location that can hold an integer, a positive or negative non-decimal number. It is denoted by the keyword, 'int'.
Use int when possible, and use Integer when needed. Since int is a primitive, it will be faster.
Convert Int to Integer Using the Integer. valueOf() Method in Java. This is another that we can use to convert an int to an Integer in Java. Here, we used valueOf() method of the Integer class.
int[] is an array of primitive int values. Integer[] is an "object" array, holding references to Integer objects. Most important practical difference: int[] cannot hold null values.
int is a primitive type. Variables of type int store the actual binary value for the integer you want to represent. int.parseInt("1") doesn't make sense because int is not a class and therefore doesn't have any methods.
Integer is a class, no different from any other in the Java language. Variables of type Integer store references to Integer objects, just as with any other reference (object) type. Integer.parseInt("1") is a call to the static method parseInt from class Integer (note that this method actually returns an int and not an Integer).
To be more specific, Integer is a class with a single field of type int. This class is used where you need an int to be treated like any other object, such as in generic types or situations where you need nullability.
Note that every primitive type in Java has an equivalent wrapper class:
byte has Byte
short has Short
int has Integer
long has Long
boolean has Boolean
char has Character
float has Float
double has Double
Wrapper classes inherit from Object class, and primitive don't. So it can be used in collections with Object reference or with Generics.
Since java 5 we have autoboxing, and the conversion between primitive and wrapper class is done automatically. Beware, however, as this can introduce subtle bugs and performance problems; being explicit about conversions never hurts.
An Integer is pretty much just a wrapper for the primitive type int. It allows you to use all the functions of the Integer class to make life a bit easier for you.
If you're new to Java, something you should learn to appreciate is the Java documentation. For example, anything you want to know about the Integer Class is documented in detail.
This is straight out of the documentation for the Integer class:
The Integer class wraps a value of the primitive type int in an object. An object of type Integer contains a single field whose type is int.
An int variable holds a 32 bit signed integer value. An Integer (with capital I) holds a reference to an object of (class) type Integer, or to null.
Java automatically casts between the two; from Integer to int whenever the Integer object occurs as an argument to an int operator or is assigned to an int variable, or an int value is assigned to an Integer variable. This casting is called boxing/unboxing.
If an Integer variable referencing null is unboxed, explicitly or implicitly, a NullPointerException is thrown.
(In the above text, the term "variable" means local variable, field or parameter)
Integer refers to wrapper type in Java whereas int is a primitive type. Everything except primitive data types in Java is implemented just as objects that implies Java is a highly qualified pure object-oriented programming language. If you need, all primitives types are also available as wrapper types in Java. You can have some performance benefit with primitive types, and hence wrapper types should be used only when it is necessary.
In your example as below.
Integer n = 9;
the constant 9 is being auto-boxed (auto-boxing and unboxing occurs automatically from java 5 onwards) to Integer and therefore you can use the statement like that and also Integer n = new Integer(9). This is actually achieved through the statement Integer.valueOf(9).intValue();
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