深入學習java原始碼之Math.sin()與 Math.sqrt()

native關鍵字

凡是一種語言，都希望是純。比如解決某一個方案都喜歡就單單這個語言來寫即可。Java平臺有個使用者和本地C程式碼進行互操作的API，稱為JNI

native關鍵字告訴編譯器（其實是JVM）呼叫的是該方法在外部定義，這裡指的是C。

java原始碼

public final class Math {

    private Math() {}

    public static final double E = 2.7182818284590452354;

    public static final double PI = 3.14159265358979323846;
	
	public static double sin(double a) {
        return StrictMath.sin(a); // default impl. delegates to StrictMath
    }
	
	public static double cos(double a) {
        return StrictMath.cos(a); // default impl. delegates to StrictMath
    }
	
    public static double tan(double a) {
        return StrictMath.tan(a); // default impl. delegates to StrictMath
    }
	
    public static double asin(double a) {
        return StrictMath.asin(a); // default impl. delegates to StrictMath
    }	
	
    public static double acos(double a) {
        return StrictMath.acos(a); // default impl. delegates to StrictMath
    }	
	
    public static double atan(double a) {
        return StrictMath.atan(a); // default impl. delegates to StrictMath
    }	
	
    public static double exp(double a) {
        return StrictMath.exp(a); // default impl. delegates to StrictMath
    }	
	
    public static double log(double a) {
        return StrictMath.log(a); // default impl. delegates to StrictMath
    }	
	
    public static double log10(double a) {
        return StrictMath.log10(a); // default impl. delegates to StrictMath
    }	
	
    public static double sqrt(double a) {
        return StrictMath.sqrt(a); // default impl. delegates to StrictMath
                                   // Note that hardware sqrt instructions
                                   // frequently can be directly used by JITs
                                   // and should be much faster than doing
                                   // Math.sqrt in software.
    }	
	
    public static double cbrt(double a) {
        return StrictMath.cbrt(a);
    }

    public static double IEEEremainder(double f1, double f2) {
        return StrictMath.IEEEremainder(f1, f2); // delegate to StrictMath
    }

    public static double atan2(double y, double x) {
        return StrictMath.atan2(y, x); // default impl. delegates to StrictMath
    }

    public static double pow(double a, double b) {
        return StrictMath.pow(a, b); // default impl. delegates to StrictMath
    }	

    public static double sinh(double x) {
        return StrictMath.sinh(x);
    }

    public static double cosh(double x) {
        return StrictMath.cosh(x);
    }

    public static double tanh(double x) {
        return StrictMath.tanh(x);
    }

    public static double hypot(double x, double y) {
        return StrictMath.hypot(x, y);
    }

    public static double expm1(double x) {
        return StrictMath.expm1(x);
    }

    public static double log1p(double x) {
        return StrictMath.log1p(x);
    }



}	

public final class StrictMath {

    private StrictMath() {}

    public static final double E = 2.7182818284590452354;

    public static final double PI = 3.14159265358979323846;

    public static native double sin(double a);

    public static native double cos(double a);

    public static native double tan(double a);

    public static native double asin(double a);

    public static native double acos(double a);

    public static native double atan(double a);

    public static strictfp double toRadians(double angdeg) {
        // Do not delegate to Math.toRadians(angdeg) because
        // this method has the strictfp modifier.
        return angdeg / 180.0 * PI;
    }

    public static strictfp double toDegrees(double angrad) {
        // Do not delegate to Math.toDegrees(angrad) because
        // this method has the strictfp modifier.
        return angrad * 180.0 / PI;
    }

    public static native double exp(double a);

    public static native double log(double a);

    public static native double log10(double a);

    public static native double sqrt(double a);

    public static native double cbrt(double a);

    public static native double IEEEremainder(double f1, double f2);

    public static native double atan2(double y, double x);

    public static native double pow(double a, double b);

    public static native double sinh(double x);

    public static native double cosh(double x);

    public static native double tanh(double x);

    public static native double hypot(double x, double y);

    public static native double expm1(double x);

    public static native double log1p(double x);
	
}