How do you know a variable type in java?

java variables types

451,174

Solution 1

a.getClass().getName()

Solution 2

Expanding on Martin's answer...

Martins Solution

a.getClass().getName()

Expanded Solution

If you want it to work with anything you can do this:

((Object) myVar).getClass().getName()
//OR
((Object) myInt).getClass().getSimpleName()

In case of a primitive type, it will be wrapped (Autoboxed) in a corresponding Object variant.

Example #1 (Regular)

private static String nameOf(Object o) {
    return o.getClass().getSimpleName();
}

Example #2 (Generics)

public static <T> String nameOf(T o) {
    return o.getClass().getSimpleName();
}

Additional Learning

Material on Java Types, Values and Variables: https://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html
Autoboxing and Unboxing: https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html
Docs on Pattern Matching for instanceof: https://docs.oracle.com/en/java/javase/14/language/pattern-matching-instanceof-operator.html

Solution 3

If you want the name, use Martin's method. If you want to know whether it's an instance of a certain class:

boolean b = a instanceof String

Solution 4

I learned from the Search Engine(My English is very bad , So code...) How to get variable's type? Up's :

String str = "test";
String type = str.getClass().getName();
value: type = java.lang.String

this method :

str.getClass().getSimpleName();
value:String

now example:

Object o = 1;
o.getClass().getSimpleName();
value:Integer

Solution 5

Use operator overloading feature of java

class Test {

    void printType(String x) {
        System.out.print("String");
    }

    void printType(int x) {     
        System.out.print("Int");
    }

    // same goes on with boolean,double,float,object ...

}

View more solutions

451,174

Miguel Ribeiro

SOreadytohelp

Updated on March 05, 2022

Comments

Miguel Ribeiro about 2 years
Let's say I declare a variable:
```
String a = "test";
```
And I want to know what type it is, i.e., the output should be java.lang.String How do I do this?
- Joachim Sauer about 14 years
  
  Are you really interested in the type of the variable? Or do you care about the type of the value? Because the type of the variable can't easily be gotten (in fact it's not possible at all for local variables and requires reflection for fields).
- Paul Tomblin about 14 years
  
  @Joachim, what exactly is the difference between "type of the variable" and "type of the value"?
- Michael Borgwardt about 14 years
  
  @Paul: Consider Object o = "o"; - the type of the variable is Object, the type of the value is String.
- Ben Lings about 14 years
  
  @Paul In List<String> l = new ArrayList<String>();, the type of the variable is List<String>, the type of the value is ArrayList<String>.
- Ajay Takur over 10 years
  
  @Ben Lings The type of variable is java.util.ArrayList and the type of value is java.util.ArrayList.
- Ben Lings over 10 years
  
  @AjayThakur - it's the difference between the compile-time (static) type and the runtime (dynamic) type.
Joachim Sauer about 14 years

That will give the type of the value. not necessarily the type of the variable.
Martin about 14 years

I just figured that was what the OP was really looking for since the declaration of a is pretty obvious at compile time
Miguel Ribeiro about 14 years

That would work if the types aren't primitives... If the type is int , how to know the type?
Joachim Sauer about 14 years

@Miguel: since the only way you can handle an int value is in an int variable, there's no way to write code that handles a int value and doesn't know that type. The matter is different in case you're handling a wrapper like Integer, but then the code of this answer works again.
Mehdi over 8 years

this is not true for a primitive type.
nicoschl over 7 years

Java does not have the concept of operator overloading, this is method overloading
skomisa about 6 years

For "but you will also get subclasses returning true...", I think you meant "but you will also get parent classes returning true...", right?
seokhoonlee almost 6 years

aren't you Martin as well? :)
chankruze over 4 years

The other Martin ... LOL ;)
Alex78191 about 4 years

Thic code Double a = 1d; boolean b = a instanceof String; will cause error error: incompatible types: Double cannot be converted to String
shevy about 4 years

I come from ruby and wanted to test: Integer.parseInt("12").getClass().getName() which evidently does not work. No idea if anyone reads this, but a universal type query method would be great. (I was trying to determine the difference between .valueOf() and .parse* related methods.)
shevy about 4 years

Interesting. I just tried your code on: System.out.println( ( (Object) Integer.parseInt("12") ).getClass().getSimpleName() ); and it works! \o/
Yossarian42 almost 4 years

@shevy Integer.parseInt() return primitive int type, not an instance of Integer class. Instead, (Object Integer.parseInt("12")) wraps the int into an Integer object, which inherits the method getClass()` from Object.
pradeep karunathilaka over 3 years

this is not given for primitive type varibales
27px over 3 years

Thank you very much, before the getSimpleName(), I tried to use (Stack)(variable.getClass().toString().split(".")).pop() and it didn't work, I was using JavaScript logic and type casted it into stack to pop the last element after spliting with . and it didn't work.
newbie almost 3 years

but this fails for a null value :(