How to check instance of a generic class?

[chan_nguyen]

Hi everyone,

I tried to check an instance of Object type with a JFS' class TreeMap<T1, T2>, but I don't know how. Could anyone help me out?

import java.util.TreeMap;


public class TreeOrHashMap 
{
	private Object instance;
	public void doSomething() 
	{
		ins = new TreeMap<Integer, Integer>();
		if( ins instanceof TreeMap<Integer, Integer> )
			TreeMap<Integer, Integer> m = ( TreeMap<Integer, Integer> )( ins );
		
		m.put( 1, 1 );
	}
}

Errors:

Exception in thread "main" java.lang.Error: Unresolved compilation problems:
ins cannot be resolved to a variable
ins cannot be resolved to a variable
Syntax error on token ")", { expected after this token
ins cannot be resolved to a variable
Syntax error, insert "}" to complete Statement

at TreeOrHashMap.doSomething(TreeOrHashMap.java:9)
at Program.main(Program.java:8)

[JosAH]

Hi everyone,

I tried to check an instance of Object type with a JFS' class TreeMap<T1, T2>, but I don't know how. Could anyone help me out?

import java.util.TreeMap;


public class TreeOrHashMap 
{
	private Object instance;
	public void doSomething() 
	{
		ins = new TreeMap<Integer, Integer>();
		if( ins instanceof TreeMap<Integer, Integer> )
			TreeMap<Integer, Integer> m = ( TreeMap<Integer, Integer> )( ins );
		
		m.put( 1, 1 );
	}
}

You forgot to define the type of reference variable 'ins', so you get compilation errors. Something like this:

Map<Integer, Integer> ins= new TreeMap<Integer, Integer>();

kind regards,

Jos

[chan_nguyen]

Hi Jos,

Thanks for your quick response. However, I think I actually define the "ins"

ins = new TreeMap<Integer, Integer>();

in this line?
Am I misunderstanding something here? If yes, could you help me point it out?

Thanks,

[JosAH]

Hi Jos,

Thanks for your quick response. However, I think I actually define the "ins"

ins = new TreeMap<Integer, Integer>();

in this line?
Am I misunderstanding something here? If yes, could you help me point it out?

Thanks,

That is not a definition; the compiler parses it as an ordinary assignment but it doesn't know the type of variable 'ins'. Read my previous reply for a proper definition of variable 'ins'.

kind regards,

Jos

[chan_nguyen]

Hi Jos again,

Sorry for being careless while copy and edit. The actual variable on that class is :

private Object ins;

instead of

private Object instance;

However, I still got error at compile time:

Exception in thread "main" java.lang.Error: Unresolved compilation problems:
Cannot perform instanceof check against parameterized type TreeMap<Integer,Integer>. Use the form TreeMap<?,?> instead since further generic type information will be erased at runtime
Syntax error on token ")", { expected after this token
Syntax error, insert "}" to complete Statement

at TreeOrHashMap.doSomething(TreeOrHashMap.java:10)
at Program.main(Program.java:8)

Another question is, is there a way to get rid of cast ( ) while calling a function from Object type, for example :

import java.util.TreeMap;


public class TreeOrHashMap 
{
	private Object ins;
	
	TreeMap<Integer, Integer> makeTreeMap()
	{
		ins = new TreeMap<Integer, Integer>();
		TreeMap<Integer, Integer> temp = ( TreeMap<Integer, Integer> )( ins );
		return temp;
	}
	
	public void doSomething() 
	{
		ins = makeTreeMap();
		//ins = new TreeMap<Integer, Integer>();
		//TreeMap<Integer, Integer> temp = ( TreeMap<Integer, Integer> )( ins );
		//temp.put( 1, 1 );
		ins.put( 1, 1 );
	}
}

If I uncomment those three lines, it works. But if I call it through an instance of type Object, it always asks me to cast "ins" to TreeMap. Is there a way work around this problem?

Thanks,

[chan_nguyen]

Hi Jos,

Thanks for clear explanation. Now I know what is "definition". My problem is that, I want to factor out the common part of TreeMap and HashMap. In other words, I want to use an instance of type object for both data structures, so I don't have duplicate code everywhere. I came from C++ background, so I was influenced strongly by C++ generic. I just try to find a way to do the same thing in Java. Could you help me out?

[JosAH]

Hi Jos,

Thanks for clear explanation. Now I know what is "definition". My problem is that, I want to factor out the common part of TreeMap and HashMap. In other words, I want to use an instance of type object for both data structures, so I don't have duplicate code everywhere. I came from C++ background, so I was influenced strongly by C++ generic. I just try to find a way to do the same thing in Java. Could you help me out?

C++ templates and Java generics are differnt beasts; Java generics are a compile time only thing while C++ templates generate code everywhere (code bloat). When you cast something the compiler can only check the cast when it is an 'up cast', i.e. if you cast a type D to type B the compiler can handle it if B is a superclass of type D. Otherwise the cast generates code for the JVM that has to handle it. Due to 'type erasure' the JVM can only check the 'raw' type, i.e. there is no way for the JVM to check the difference (there isn't any) between, say, Map<Integer, Integer> and Map<Double, Double>. I consider that a severe limitation of generics; otoh I consider the way those C++ template code generation quite clumsy too; you can't leave that all to those poor linkers ;-)

kind regards,

Jos

[chan_nguyen]

Hi Jos,

Thanks for your clear explanation. I think I got it ;)