Java（如果不是）在lambda表达式中

Question

I am trying to use a lambda expression to evaluate to true or false on whether a input parameter to a method has a certain character '*' at the end of the string; as an input to a constructor to an object. im sure this is way off, but this is what i was trying to do before i looked around the web and saw some referring to using streams, but I'm not sure how or if they would work for this situation:

public void addPermission(String permission, String resource){
    permissions.put(new Permission(permission, 
()-> {if (permission.charAt(permissions.size() - 1) == '*') return true; }));
}

any input would be greatly appreciated.

Answer 1

Do you need to use streams for this? Homework, bet, whatever?

If you don't

A plain old checking on the String with the endsWith() method would suffice.

public void addPermission(String permission, String resource){
    permissions.put(new Permission(permission, permission.endsWith("*")));
}

If you want to play around with lambdas

You can provide a way to tell the constructor what type of checking it needs to do in order to fill in that flag.

Constructor:

public Permission(String permission, Predicate<String> checker) {
    this.permission = permission;
    this.flag = checker.test(permission);
}

Caller:

new Permission(permission, (p) -> p.endsWith("*"));

Answer 2

Your lambda expression is missing an else case (or a final return), see this example:

public void addPermission(String permission, String resource){
  permissions.put(new Permission(permission, ()-> {
    if (permission.charAt(permissions.size() - 1) == '*') 
      return true;
    return false;
  }));
}

Note that this would not compile if permissions is a Map, because the signature of Map.put is not compliant.

Another simplification (beyond those already mentioned by others):

public void addPermission(String permission, String resource){
  permissions.put(new Permission(permission, ()-> permission.charAt(permissions.size() - 1) == '*'));
}

The type of the given lambda should be a FunctionalInterface with no argument and result Boolean , eg Supplier<Boolean> .