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Java Convert String to Double

將 String 轉換為 double 或 Double 是 Java 中常見的操作，可以使用 parseDouble 、 valueOf 等方法進行操作， Java String Double 任務是將該字串轉換為雙精度浮點數，本篇增加了範例，並透過單元測試來驗證產出結果。

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- conversion
       |                   +- string
       |                       +- StringToDoubleTest.java

單元測試

Java String to Double 提供字串轉換成雙精度浮點數等操作。

parseDouble

Java String to Double 使用 Double.parseDouble 轉換成雙精度浮點數，此方法接受另一個參數來更改基數，例如 16（十六進制）或 2（二進制）。

	@Test
	public void parseDouble() {
		String value = "460.88";
		double result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(460.88, result, 0);

		value = "-50.88";
		result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(-50.88, result, 0);
	}

460.88
-50.88

parseDoubleThrowException

Java String to Double 使用 Double.parseDouble 轉換成雙精度浮點數，輸入不合格式的字串，會拋出例外。

	@Test(expected = NumberFormatException.class)
	public void parseDoubleThrowException() {
		String value = "460.88";
		double result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(460.88, result, 0);

		value = "bash-460.88";
		result = Double.parseDouble(value);
		System.out.println(result);
	}

460.88
java.lang.NumberFormatException: For input string: "bash-460.88"
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.lang.Double.parseDouble(Double.java:538)
	at org.ruoxue.java_147.string.StringToDoubleTest.parseDoubleThrowException(StringToDoubleTest.java:30)

valueOf

Java String to Double 使用 Double.valueOf 轉換成雙精度浮點數。

	@Test
	public void valueOf() {
		String value = "460.88";
		Double result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(460.88, result.doubleValue(), 0);

		value = "-50.88";
		result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(-50.88, result.doubleValue(), 0);
	}

460.88
-50.88

valueOfThrowException

Convert String to Double in Java 使用 Double.valueOf 轉換成雙精度浮點數，輸入不合格式的字串，會拋出例外。

	@Test(expected = NumberFormatException.class)
	public void valueOfThrowException() {
		String value = "460.88";
		Double result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(460.88, result.doubleValue(), 0);

		value = "bash-460.88";
		result = Double.valueOf(value);
		System.out.println(result);
	}

460.88
java.lang.NumberFormatException: For input string: "bash-460.88"
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.lang.Double.parseDouble(Double.java:538)
	at java.lang.Double.valueOf(Double.java:502)
	at org.ruoxue.java_147.string.StringToDoubleTest.valueOfThrowException(StringToDoubleTest.java:55)

StringToDoubleTest.java

Convert String to Double in Java 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.conversion.string;

import static org.junit.Assert.*;

import org.junit.Test;

public class StringToDoubleTest {

	@Test
	public void parseDouble() {
		String value = "460.88";
		double result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(460.88, result, 0);

		value = "-50.88";
		result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(-50.88, result, 0);
	}

	@Test(expected = NumberFormatException.class)
	public void parseDoubleThrowException() {
		String value = "460.88";
		double result = Double.parseDouble(value);
		System.out.println(result);
		assertEquals(460.88, result, 0);

		value = "bash-460.88";
		result = Double.parseDouble(value);
		System.out.println(result);
	}

	@Test
	public void valueOf() {
		String value = "460.88";
		Double result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(460.88, result.doubleValue(), 0);

		value = "-50.88";
		result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(-50.88, result.doubleValue(), 0);
	}

	@Test
	public void valueOfThrowException() {
		String value = "460.88";
		Double result = Double.valueOf(value);
		System.out.println(result);
		assertEquals(460.88, result.doubleValue(), 0);

		value = "bash-460.88";
		result = Double.valueOf(value);
		System.out.println(result);
	}
}

心得分享

String to Double in Java 轉換方法 parseDouble 和 valueOf 似乎完全相同，分別傳回基本型別 double 及物件 Double ， valueOf 方法在內部委託給 parseDouble 方法， Convert String to Double in Java 提供將 String 實例轉換為 double 或 Double 實例的多種方法。