DecimalFormat (when converting double to String, the data is too small or too large to automatically convert to scientific notation)

background:

In the project we are doing, there is a string generation, we need double to splicing, and it turns out that after splicing the string, the double value uses scientific notation (scientific notation is used by scientists to represent very large or very A convenient method for small numbers that satisfy the regular expression [±][1-9]"."[0-9]+E[±][0-9]+, that is, the integer part of the number is only 1 digits, the fractional part has at least 1 digit, and the sign of the number and its exponent part must be clearly given even for positive numbers.) expressed.

I tried it with int and long, and found that int and long do not have this problem, only double and float including their respective packaging classes (Double and Float) also have this problem. And it is found that when the number of double type to String type is too small or too large, it will be automatically converted to scientific notation. (No problem when it is 0.00xxxx, it will automatically convert to scientific notation when it is 0.000xxxxx, no problem when it is 1000000, and it will automatically convert to scientific notation when it is 10000000.)

How to prevent scientific notation from appearing in the result? It is found that the result of double and float needs to be formatted in decimal format , so that the conversion into a string will not use scientific notation.

    public static void main(String[] args) {
        //int类型
        int intValue1 = 1;
        //Integer包装类
        Integer intValue2 = 12700000;
        //long类型
        long longValue1 = 1L;
        long longValue2 = 1000000000L;
        //Long包装类
        Long longValue3 = 1L;
        Long longValue4 = 1000000000L;
        //float类型
        float floatValue1 = 0.001234F;
        float floatValue2 = 0.00012345F;
        //Float包装类
        Float floatValue3 = 1234567.0F;
        Float floatValue4 = 12345678.0F;
        //double类型
        double doubleValue1 = 0.001234;
        double doubleValue2 = 0.00012345;
        //double包装类
        Double doubleValue3 = 1234567.00;
        Double doubleValue4 = 12345678.00;
        //
        DecimalFormat df = new DecimalFormat("0.############");//#代表小数点后如果没有 就为空

        System.out.println(
            "处理前:\n" +    
            "int1:" + intValue1 + "\n" +
            "int2:" + intValue2 + "\n" +
            "long1:" + longValue1 + "\n" +
            "long2:" + longValue2 + "\n" +
            "long3:" + longValue3 + "\n" +
            "long4:" + longValue4 + "\n" +
            "float1:" + floatValue1 + "\n" +
            "float2:" + floatValue2 + "\n" +
            "float3:" + floatValue3 + "\n" +
            "float4:" + floatValue4 + "\n" +
            "double1:" + doubleValue1 + "\n" +
            "double2:" + doubleValue2 + "\n" +
            "double3:" + doubleValue3 + "\n" +
            "double4:" + doubleValue4);

        System.out.println(
            "处理后:\n" +
            "dfFloat1:" + df.format(floatValue1) + "\n" +
            "dfFloat2:" + df.format(floatValue2) + "\n" +
            "dfFloat3:" + df.format(floatValue3) + "\n" +
            "dfFloat4:" + df.format(floatValue4) + "\n" +
            "dfDouble1:" + df.format(doubleValue1) + "\n" +
            "dfDouble2:" + df.format(doubleValue2) + "\n" +
            "dfDouble3:" + df.format(doubleValue3) + "\n" +
            "dfDouble4:" + df.format(doubleValue4));
    }

The result is as follows:

DecimalFormat usage

DecimalFormat is a concrete subclass of NumberFormat for formatting decimal numbers.

DecimalFormat contains a pattern and a set of symbols

Symbol meaning:

  0 一个数字 
  # 一个数字，不包括 0 
  . 小数的分隔符的占位符 
  , 分组分隔符的占位符 
  ; 分隔格式。 
  - 缺省负数前缀。 
  % 乘以 100 和作为百分比显示 
  ? 乘以 1000 和作为千进制货币符显示；用货币符号代替；如果双写，用 国际货币符号代替。如果出现在一个模式中，用货币十进制分隔符代 替十进制分隔符。 
  X 前缀或后缀中使用的任何其它字符，用来引用前缀或后缀中的特殊字符。 

1. Keep two decimal places (truncated)

    double num = 11.256;
    DecimalFormat df = new DecimalFormat("#.##");
    //指定RoundingMode
    df.setRoundingMode(RoundingMode.DOWN);
    String str = df.format(num);
    double formatNum = Double.parseDouble(str);
    System.out.println(formatNum);

2. Keep two decimal places (rounded)

    double num = 11.256;
    DecimalFormat df = new DecimalFormat("#.##");
    //指定RoundingMode
    df.setRoundingMode(RoundingMode.HALF_UP);
    String str = df.format(num);
    double formatNum = Double.parseDouble(str);
    System.out.println(formatNum);

3. Commonly used RoundingMode

HALF_EVEN -----DecimalFormat默认的RoundingMode为RoundingMode.HALF_EVEN，也称为“银行家舍入法”，主要在美国使用。四舍六入，五分向相邻的偶数舍入。
以下例子为保留小数点1位，那么这种舍入方式下的结果。
1.15–>1.2 1.25–>1.2

HALF_UP — 四舍五入

HALF_DOWN — 五舍六入
2.5 —> 2
2.6 —> 3

DOWN — 直接截断

UP — 始终对非零舍弃部分前面的数字加 1

4. Placeholders

0表示数字占位符，实际位数不够时补零
比实际数字的位数多，不足的地方用0补上。
new DecimalFormat(“00.00”).format(3.14) //结果：03.14
new DecimalFormat(“0.000”).format(3.14) //结果： 3.140
new DecimalFormat(“00.000”).format(3.14) //结果：03.140
整数部分比实际数字的位数少，整数部分不改动
小数部分比实际数字的位数少，根据小数部分占位符数量保留小数
new DecimalFormat(“0.000”).format(13.146) //结果：13.146
new DecimalFormat(“00.00”).format(13.146) //结果：13.15
new DecimalFormat(“0.00”).format(13.146) //结果：13.15
#表示数字占位符，实际位数不够时不补零
比实际数字的位数多，不变
new DecimalFormat("##.##").format(3.14) //结果：3.14
new DecimalFormat("#.###").format(3.14) //结果： 3.14
new DecimalFormat("##.###").format(3.14) //结果：3.14
整数部分比实际数字的位数少，整数部分不改动
小数部分比实际数字的位数少，根据小数部分占位符数量保留小数
new DecimalFormat("#.###").format(13.146) //结果：13.146
new DecimalFormat("##.##").format(13.146) //结果：13.15
new DecimalFormat("#.##").format(13.146) //结果：13.15

Common examples:

    public static void main(String[] args) {
        DecimalFormat df = new DecimalFormat();
        double data = 1234.56789;
        System.out.println("格式化之前的数字: " + data);
        String style = "0.0";//定义要显示的数字的格式
        df.applyPattern(style);// 将格式应用于格式化器
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        style = "00000.000 kg";//在格式后添加诸如单位等字符
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的"#"表示如果该位存在字符，则显示字符，如果不存在，则不显示。
        style = "##000.000 kg";
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的"-"表示输出为负数，要放在最前面
        style = "-000.000";
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的","在数字中添加逗号，方便读数字
        style = "-0,000.0#";
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的"E"表示输出为指数，"E"之前的字符串是底数的格式，
        // "E"之后的是字符串是指数的格式
        style = "0.00E000";
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的"%"表示乘以100并显示为百分数，要放在最后。
        style = "0.00%";
        df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df.format(data));
        // 模式中的"\u2030"表示乘以1000并显示为千分数，要放在最后。
        style = "0.00\u2030";
        //在构造函数中设置数字格式
        DecimalFormat df1 = new DecimalFormat(style);
        //df.applyPattern(style);
        System.out.println("采用style: " + style + "  格式化之后: " + df1.format(data));
    }

The result display: