Java StringBuilder字符追加返回不需要的数字

Question

I am very sorry if this is a basic question which has been answered before (I tried looking but I did not find anything)

I am trying to write the following Java method:

 String winningCard(String trick, char trump) {
    StringBuilder sb = new StringBuilder();
    char suit;
    char rank;
    for(int i = 1;   i < trick.length(); i+=2) {
        if(trick.charAt(i) == trump) {
            suit = trick.charAt(i);
            rank = trick.charAt(i-1);
            sb.append(rank + suit); //issue here, returns a weird number
            break;
        }
    }
    String result = sb.toString();
    return result;
}

When called with these arguments "8s7hQd", 'h' for example, it is supposed to return "7h".

If I change the StringBuilder to only append either the suit or the rank, it does it just fine, but if I put it the way it is above it returns "159" which I believe has something to do with the unicode encoding.

I'd very much appreciate if a kind sould could tell me what I am missing.

Thanks in advance

Answer 1

suit and rank are basically numbers. The + is adding these numbers and appending it.

If you place a "" between, the chars will be appended as you intend, because it forces the compiler to use the + with a String.

sb.append(rank + "" + suit);

Answer 2

append(rank).append(suit);

应该做的把戏

Answer 3

+ is a tricksy thing, because it means different things in different contexts.

• If at least one of the operands is a String , it acts as the string concatenation operator.
• If both of the operands are numbers, or convertible to numbers via unboxing, then it acts as the numeric addition operator.

You are giving it two char s: these are numbers, so numeric addition occurs.

Before adding the two chars, they are widened to int ; the result is an int too. And it is this int that you are appending to the string builder, hence the "unwanted" number.

So, either avoid using the addition operator at all (best):

sb.append(rank).append(suit);

Or make sure you are using the string concatenation operator:

sb.append("" + rank + suit);
// Left-associative, so evaluated as
// ("" + rank) + suit

sb.append(String.valueOf(rank) + suit);
// Etc.

---

But actually, you don't need to do either: just append the substring:

sb.append(trick, i-1, i+1);

This extracts a portion of the trick string, as trick.substring(i-1, i+1) would, but does it without creating a new string.

And you don't need a loop

Answer 4

您可以直接说这些字符应通过以下方式解释为String

sb.append(String.valueOf(rank) + String.valueOf(suit))