Why Does Java Throw an 'Incompatible types: void cannot be converted to ...' Compilation Error?

What does "Incompatible types: void cannot be converted to ..." mean?

Problem:

When compiling Java code, programmers might encounter compilation errors such as:

• "Incompatible types: void cannot be converted to ..."
• "Type mismatch: cannot convert from void to ..."
• "Incompatible types. Required: ... found: void"
• "Incompatible types: Found void, required ..."

Meaning:

These error messages indicate a type mismatch in the code, specifically involving the "void" type. Void is not a primitive or reference type in Java. It represents a method that does not return a value.

Fix:

To resolve this error, determine the method's intended usage:

1. Review method documentation (Javadocs): Check the documentation for the method being called to understand its return type.
2. Correct code usage: Adjust the method call to either use the result (if a return value is expected) or remove the attempt to assign the result.
3. Modify method signature (Optional): If the method is intended to return a value, modify its signature to reflect the correct return type.

Example:

Consider the following code:

public class Test {
    private static void add(int a, int b) {
        int res = a + b;
    }

    public static void main(String[] args) {
        int sum = add(1, 1);
    }
}

This code generates the error "incompatible types: void cannot be converted to int" because the add method does not return a value, but the code attempts to assign the result to a variable. To fix the error, either modify the method signature to return an int or remove the assignment statement.

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