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I suggest learning Java fundamentals first. Declaring a variable is fundamental to Java programming language (and indeed many programming languages), its not just Android specific.
You can learn Java fundamentals here.
Good Luck in your learning
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public class MyApplication extends Application {
private String someVariable;
public String getSomeVariable() {
return someVariable;
}
public void setSomeVariable(String someVariable) {
this.someVariable = someVariable;
}
}