在while循环中返回一个字符串

在这里，我想避免使用该行return "";，而是返回我输出的所有值System.out.println();

我尝试使用一种StringBuilder方法，但是当我运行测试程序时，此方法不会输出任何内容。我猜想它StringBuilder必须在while循环中以不同的方式工作，但是我找不到方法！

任何建议或指导将不胜感激！

public String toString(){
    int topLine = size;
    int topCurrentLine = size;
    int bottomLine = 1;
    int bottomCurrentLine = size;

    while(topCurrentLine<=size){
        topCurrentLine= (size-topLine)+1;

        if (topCurrentLine<=size){
            for (int i = 0; i < (topCurrentLine-1); ++i){
                System.out.print(".");
            }

            System.out.print(starMinusLine(getSandInLine(topLine), ((2*topLine-1)-getSandInLine(topLine)),false));

            for (int i = 0; i < (topCurrentLine-1); ++i){
                System.out.print(".");
            }
            topLine = topLine -1;

            System.out.print("\n");
        }       
    }

    while(bottomCurrentLine>1){         
        bottomCurrentLine= (size-bottomLine)+1;

        for (int i = 0; i < (bottomCurrentLine-1); ++i){
            System.out.print(".");
        }

        System.out.print(starMinusLine(getSandInLine(bottomLine), ((2*bottomLine-1)-getSandInLine(bottomLine))));

        for (int i = 0; i < (bottomCurrentLine-1); ++i){
            System.out.print(".");
        }
        bottomLine = bottomLine +1;

        System.out.print("\n");     
    }

    return "";      
}

这是我对StringBuilder的尝试：

public String toString(){ 
    int topLine = size;
    int topCurrentLine = size;
    int bottomLine = 1;
    int bottomCurrentLine = size;

    StringBuilder y = new StringBuilder();

    while(topCurrentLine<=size) {
        topCurrentLine= (size-topLine)+1;

        if (topCurrentLine<=size){
            for (int i = 0; i < (topCurrentLine-1); ++i){
                y.append(".");
            }

            y.append(starMinusLine(getSandInLine(topLine), ((2*topLine-1)-getSandInLine(topLine)),false));

            for (int i = 0; i < (topCurrentLine-1); ++i){
                y.append(".");
            }
            topLine = topLine -1;

            y.append("\n");
        }

    }

    while(bottomCurrentLine > 1) {      
        bottomCurrentLine= (size-bottomLine)+1;

        for (int i = 0; i < (bottomCurrentLine-1); ++i){
            y.append(".");
        }

        y.append(starMinusLine(getSandInLine(bottomLine), ((2*bottomLine-1)-getSandInLine(bottomLine))));

        for (int i = 0; i < (bottomCurrentLine-1); ++i){
            y.append(".");
        }
        bottomLine = bottomLine +1;

        y.append("\n");
    }

    return y.toString();
}