Declare a final variable based on a condition and use it in lambda in java

Question

I have the below scenario.

I would like to use a variable inside lambda expression. But this variable will be have a onetime value(final) based on a condition.

  final String constraintsAmount;
  if(constraint.isPresent()) {
       constraintsAmount = constraint.getValue();
  }

After this I start iterating over a list using forEach + lambda expression. Now I have to use this constraintsAmount field inside this iteration.

But it says that "constraintsAmount might not have been initialized".

How can I get around this one.

Note :

I don't want to declare this variable as an instance variable and I certainly don't want to declare and initialize this variable inside the iteration.
Since it is a final I cant initialize it and then reuse it inside the if check. So wanted to check what is the work around.

@BoristheSpider Yes I understand why it is giving the error, since if constraint is not present variable wont be initialized. So what is the work around to solve this. — BigDataLearner, Oct 04 '17 at 21:19
@tushR we can't tell you that. You have to decide what you _want_ to happen when `constraint` isn't present. You have't told us that. — Louis Wasserman, Oct 04 '17 at 21:27

score 5 · Accepted Answer · answered Oct 04 '17 at 21:19

From a compiler perspective you need an else block:

final String constraintsAmount;
  if(constraint.isPresent()) {
    constraintsAmount = constraint.getValue();
  } else {
    constraintsAmount = ...
  }

Even better the ternary operator:

final String constraintsAmount = constraint.isPresent() ? constraint.getValue() : ...

NOTE: In Java 8 you don't need to declare a variable final to use it in a "closure" (inside lambda stuff).

Or maybe you might need to redesign your code altogether. Hard to tell as I don't have enough information.

score 1 · Answer 2 · answered Oct 04 '17 at 21:22

1

Assumimg constraint is an Optional<String> you could do the following:

String constraintsAmount = constraint.orElse(<somedefault>);

Otherwise if you only want to do something if constraint has a value do the following:

constraint.ifPresent(constraintsAmount -> { .... });

answered Oct 04 '17 at 21:22

M. le Rutte

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(in the last case you obviously do not need to declare the `constraintsAmount` variable.) – M. le Rutte Oct 04 '17 at 21:24

davidxxx · Answer 3 · 2017-10-04T21:28:38.877

A local variable has to be explicitly initialized before being used.
Your problem is not specific to the final modifier or the use of a lambda expression.

If this condition is true : if(constraint.isPresent()) {, constraintsAmount is initialized. But in the contrary case, it is not initialized. So if you use this variable, the compiler doesn't accept that.

This code illustrates the problem :

  final String constraintsAmount;
  if(constraint.isPresent()) { // initialized
       constraintsAmount = constraint.getValue();
  }
  // if the conditional statement is false, the constraintsAmount var is not initialized

  // So the compiler generates the error 
  // constraintsAmount might not have been initialized

  String lowerCaseString = constraintsAmount.toLowerCase();

Either, value constraintsAmount in an else statement.
Or remove final to give a default value to the variable.

Declare a final variable based on a condition and use it in lambda in java

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