Question

(Using Java) I am implementing a generic class which is a B-Tree. When the user runs the program they can supply some arguments which will determine the type of the tree (Integer, Character, Double or String).

In my main method I have this code:

// Get user input and split it into tokens
// Tokens[1] = the type specified by the user

if( tokens[1].equals("DOUBLE"))
    BTree<Double> t = new BTree<Double>();

else if( tokens[1].equals("CHARACTER"))
    BTree<Character> t = new BTree<Character>();

else if( tokens[1].equals("INTEGER"))
    BTree<Integer> t = new BTree<Integer>();

else if( tokens[1].equals("STRING"))
    BTree<String> t = new BTree<String>();

But the compiler complains if I have the if statements. If I remove them then it compiles and runs fine :/ how can I fix this? So that the user can supply the type and the tree will be created depending on the type supplied? Thanks.

Here is some of the compiler output:

C:\Users\User\Desktop>javac *.java
Main.java:42: error: cannot find symbol
BTree<Double> t = new BTree<Double>();
symbol:   variable BTree
location: class Main

Main.java:42: error: cannot find symbol
BTree<Double> t = new BTree<Double>();
symbol:   variable Double
location: class Main

Main.java:42: error: cannot find symbol
BTree<Double> t = new BTree<Double>();
symbol:   variable t
location: class Main

.. There is more but it is similar and repeated for each of the types

Answer 1

The variable t in each of the if statements is only defined in the if‘s context, outside them the compiler won’t recognize them.
Define your variable t before the if statements.

BTree<?> t = null;
if( tokens[1].equals("DOUBLE")) {
    t = new BTree<Double>();
}
else if( tokens[1].equals("CHARACTER")) {
    t = new BTree<Character>();
}
else if( tokens[1].equals("INTEGER")) {
    t = new BTree<Integer>();
}
else if( tokens[1].equals("STRING")) {
    t = new BTree<String>();
}
// now you can use your 't'