Question

我正在学习Lambdas并且在转换方面遇到了一些困难。我需要引入一个List，使用类Arrays的asList方法复制Field类的values方法提供的数组。然后我需要使用lambda表达式作为参数，使用forEach内部循环转换for循环。 lambda表达式的主体将是for循环的当前主体的代码。我相信我的List语法是正确的（List list = Arrays.asList（data）;），但是我很难弄清楚如何处理for循环，甚至从哪里开始。任何指导将不胜感激。感谢

public AreaData(String... data)
{
    List<String> list = Arrays.asList(data);


    /* Assert to check that the data is of the expected number of items. */
    assert data.length == Field.values().length : "Incorrect number of fields";

    for( Field field : Field.values() )
    {
        int width;
        String formatString;
        if( field == NAME )
        {
            /* Get the name value and store it away. */
            String value = data[field.position()];
            strings.put(field, value);
            /* Get the needed width of the field to hold the name. */
            width = max(value.length(), field.getFieldHeading().length());
            formatString = "s";
        } else
        {
            /* If the value is of the wrong form, allow the NumberFormatException
               to be thrown. */
            Double value = Double.parseDouble(data[field.position()]);
            /* Assertion to check value given is positive.  */
            assert value.compareTo(0.0) >= 0 :
                    "invalid " + field.name() + " value=" + value.toString();
            /* Get the field value and store it away. */
            doubles.put(field, value);
            /* Get needed width of the field to hold the heading or value. */
            width = max((int) log10(value) + MINIMUM,
                    field.getFieldHeading().length() + HEADING_SEPARATION);
            formatString = ".2f";
        }
        /* Keep the widest value seen, and record the corresponding format. */
        if( width > WIDTHS.get(field) )
        {
            WIDTHS.put(field, width);
            FORMATS.put(field, "%" + width + formatString);
        }
    }
    /* Optimization: to avoid doing this every time a comparison is made. */
    this.nameCaseless = strings.get(NAME).toUpperCase().toLowerCase();
}

Answer 1

Stream.of(Field.values()).forEach()应该这样做：

public AreaData (String... data) {
        List<String> list = Arrays.asList(data);
        /* Assert to check that the data is of the expected number of items. */
        assert data.length == Field.values().length : "Incorrect number of fields";
        int width;
        String formatString;
        Stream.of(Field.values()).forEach(
                field -> {
                    if (field == NAME) {
                        /* Get the name value and store it away. */
                        String value = data[field.position()];
                        strings.put(field, value);
                        /* Get the needed width of the field to hold the name. */
                        width = max(value.length(), field.getFieldHeading().length());
                        formatString = "s";
                    } else {
                        /* If the value is of the wrong form, allow the NumberFormatException
                           to be thrown. */
                        Double value = Double.parseDouble(data[field.position()]);
                        /* Assertion to check value given is positive.  */
                        assert value.compareTo(0.0) >= 0 :
                                "invalid " + field.name() + " value=" + value.toString();
                        /* Get the field value and store it away. */
                        doubles.put(field, value);
                        /* Get needed width of the field to hold the heading or value. */
                        width = max((int) log10(value) + MINIMUM,
                                field.getFieldHeading().length() + HEADING_SEPARATION);
                        formatString = ".2f";
                    }
                    /* Keep the widest value seen, and record the corresponding format. */
                    if (width > WIDTHS.get(field)) {
                        WIDTHS.put(field, width);
                        FORMATS.put(field, "%" + width + formatString);
                    }
                });

        /* Optimization: to avoid doing this every time a comparison is made. */
        this.nameCaseless = strings.get(NAME).toUpperCase().toLowerCase();
    }

那就是说，你应该考虑以下经验法则：

一个lambda表达式最多应该是3行代码，而不是案件超过5行！

Answer 2

如果您特别想将此转换为使用stream和lambdas，那么我觉得您也应该借此机会根据这些工具的意图进行重构。这意味着使用过滤器，收集器等，而不是仅将所有代码转换为单个lambda。

例如：

Arrays.stream(Field.values())
    .peek(field -> field.storeValue(data))
    .filter(field -> field.getWidth(data) > widths.get(field))
    .forEach(field -> storeWidthAndFormat(data, widths, formats));

这假设您将与NAME关联的逻辑封装在Field枚举中（这是我建议的）。

将for循环转换为forEach Lambda

2 个答案: