为什么 eclipse 说我的方法没有返回有效的结果？

Question

I've been doing this code in java for a Sudoku game since a while and I don't know what's wrong, Maybe is the "if" or de "For", but the IDE says that my method doesn't return a booelean type.

// check if the number has already been used in the columns
private boolean checkColumns(int x, int y, int value) {
    for (int j = 0; j < 9; j++) {
        if (this.gridPlayer[j][y].getValue() == value) return false;
        else return true;
    }

    }
// Check if the number has already been used in the lines
private boolean checkLines(int x, int y, int value) {
    for (int i = 0; i <= 9; i++) {
        if (this.gridPlayer[x][i].getValue() == value) return false;
         else return true;
    }
    }

// Check if the number has already been used and the subGrid
private boolean checkSubGrid(int x, int y) {
    for (int i = 0; i <= 9; i++) {
        for (int j = 0; j <= 9; j++) {
            if (this.gridPlayer[x][y].getValueOfSubGrid(x, y) == this.gridPlayer[i][j].getValueOfSubGrid(i, j)) {
                if (this.gridPlayer[x][y].getValue() == this.gridPlayer[i][j].getValue()) {
                    return false;
                } else {
                    return true;
                }
            } else if (this.gridPlayer[x][y].getValueOfSubGrid(x, y) != this.gridPlayer[i][j].getValueOfSubGrid(i,
                    j)) {
                return true;
            }
        }
    }
}

Answer 1

The compiler is assuming that it is not 100% sure that the return statement from within your "for" loops will be called so it is seeing a path where your methods do not return any values even though they declare they do.

You need to have some return value outside of your loops, even if you are sure this will never happen ie

private boolean checkLines(int x, int y, int value) {
  for (int i = 0; i <= 9; i++) {
    if (this.gridPlayer[x][i].getValue() == value) return false;
     else return true;
  }
 return false; //even if you think it will never be run it is necessary 
}

Answer 2

Welcome,
in your checkSubGrid() method you need to return a value if the runtime dont enter in the last else if : else if (this.gridPlayer[x][y]...) {

If the method is not void , you need to put a return.

 if(a > 1) {
   return a;
 } else {
   return b;
 }

In this case above we have a if - else statement, the method will always return true or false(or have a exception).

 if(a > 1) {
   return a;
 } else if(a == 0) {
   return b;
 }

In other hand, the method can or can not enter in the second if , them you have no returns. You are not making sure that the compiler will have a return.

You can solve this putting a default return, or putting a else statement.

 if(a > 1) {
   return a;
 } else if(a == 0) {
   return b;
 } else {
   return null;
 }

Or

 if(a > 1) {
   return a;
 } else if(a == 0) {
   return b;
 }
 return null;