The left-hand side of an assignment must be a variable エラーが出る

以下のプログラムで、何故エラーが出てしまうのでしょうか。

▪️Test85.java

import java.util.function.*;

public class Test85 {
    public static void main(String[] args) {
//      Hero85 h = new Hero85();
        (Cat85 c) -> {
            return c.getHp();
            }
    }
}

▪️Cat85.java

public interface Cat85 {
    public int hp;

    Cat85() {
        this.hp = 100;
    }

    public  abstract int getHp() {
        return this.hp;
    }

}

▪️Error Log

The left-hand side of an assignment must be a variable
Multiple markers at this line
    - Syntax error, insert "AssignmentOperator Expression" to
     complete Expression
    - Syntax error, insert ";" to complete BlockStatements

ご回答お願いします