方法一：使用BigDecimal

public class BigDecimalDemo {
	public static void main(String[] args) {
		BigDecimal decimal = new BigDecimal(Math.PI);
		decimal = decimal.setScale(10, BigDecimal.ROUND_HALF_EVEN);
		System.out.println(decimal);
		decimal = decimal.setScale(5, BigDecimal.ROUND_HALF_EVEN);
		double num = decimal.doubleValue();
		System.out.println(decimal);
		System.out.println(num);
	}
}
 

執行結果：

3.1415926536
3.14159
3.14159

public class DecimalFormatDemo {
	public static void main(String[] args) {
		DecimalFormat df = new DecimalFormat("0.000");
		double num = Math.PI;
		String numStr = df.format(num);
		System.out.println(num);
		System.out.println(numStr);
	}
}
 

3.141592653589793
3.142