Using StringBuffer and Generics

Question

I was trying to use StringBuffer and Generics, but the code won't compile:

[Gen.java]

class Gen<T> 
{
T ob;
//Constructor
Gen (T o)
{
 ob = o;
}

void showtype()
{
    System.out.println("Type of T is:"+ob.getClass().getName());
}

T getOb()
{
    return ob;
}

}

[GenDemo.java]

public class GenDemo
{
public static void main(String[] args)
{

    Gen<StringBuffer> objStr;
    objStr = new Gen<StringBuffer> ("Hello"); //doesn't compile here.
    objStr.showtype();
    StringBuffer str = objStr.getOb();
}
}

I am a beginner. So, I apologize if this question is too basic for you. Can someone please help me? The code compiles well if I replace "StringBuffer" with "String"

Thanks

Why do you think it should compile? What surprises you about the error message? What do you think the type StringBuffer is? What's its relation with String? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Dec 24, 2015 at 22:56
Please don't use StringBuffer. It was replaced by StringBuilder more than ten years ago. — Peter Lawrey
– Peter Lawrey, Commented Dec 24, 2015 at 22:59
@KickButtowski if it doesn't compile, there won't be a stack trace (and it shouldn't compile) — Peter Lawrey
– Peter Lawrey, Commented Dec 24, 2015 at 23:02
@KickButtowski this doesn't compile, so you will get a compiler error, which doesn't include a stack trace. — Peter Lawrey
– Peter Lawrey, Commented Dec 26, 2015 at 14:19

Peter Lawrey · Accepted Answer · 2015-12-24 23:02:18Z

3

You can't assign between incompatible types in Java. e.g. you can't do this

StringBuilder sb = "Hello"; // the object String is not a StringBuilder

What you can do is pass a reference of one type to the constructor of another e.g.

StringBuilder sb = new StringBuilder("Hello");

and you could do

Gen<String> str = new Gen<>("Hello");

or

Gen<StringBuilder> sb = new Gen<>(new StringBuilder("Hello"));

However, Java doesn't support implicit conversions of objects or references.

answered Dec 24, 2015 at 23:02

Peter Lawrey

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Comments

sisyphus · Accepted Answer · 2015-12-24 23:00:15Z

1

You're trying to initialise a Gen<StringBuffer> with a String. Change the initialisation to objStr = new Gen<StringBuffer>(new StringBuffer("Hello")) and everything should be fine.

answered Dec 24, 2015 at 23:00

sisyphus

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1 Comment

watchtower Over a year ago

Perfect! Thanks sisyphus. Also, I am new to this forum...So I clicked up arrow to give you a vote, but it shows "-1"--I am not sure whether this is equivalent to "Dislike". I sincerely apologize if it is. I am not familiar with stackoverflow. This was my first post.

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