深入學習java原始碼之Math.ulp()與 Math.signum()
深入學習java原始碼之Math.ulp()與 Math.signum()
swtich()變數型別只能是int、short、char、byte和enum型別(JDK 1.7 之後,型別也可以是String了)。當進行case判斷時,JVM會自動從上到小掃描,尋找匹配的case,可能存在以下情況:
當每一個case都不存在break時,匹配成功後,從當前case開始,依次返回後續所有case的返回值。
int i = 2; switch(i){ case 0: System.out.println("0"); case 1: System.out.println("1"); case 2: System.out.println("2"); default: System.out.println("default"); } 輸出:2 default
若當前匹配成功的case不存在break,則從當前case開始,依次返回後續case的返回值,直到遇到break,跳出判斷。
int i = 2; switch(i){ case 0: System.out.println("0"); case 1: System.out.println("1"); case 2: System.out.println("2"); case 3: System.out.println("3");break; default: System.out.println("default"); } 輸出:2 3
default的運用,是當switch語句裡,所有的case語句都不滿足條件時,則執行default語句
default在switch開頭:
若所有case都不滿足條件,則執行default語句,並執行default語句之後的case語句,直到break或結束
default在switch中間:(同上)
若所有case都不滿足條件,則執行default語句,並執行default語句之後的case語句,直到break或結束
default在switch末尾:
若所有case語句都不滿足條件,則執行default語句,結束;若有case滿足,則執行case語句直到遇到break或switch語句結束,所以default在最後一行時break可以省略不寫(但是不建議省略,以求嚴謹)
switch語句當中,用於終止語句
getDetail:function(type,Id){
var param = {
id:Id
}
var url = "";
switch(type)
{
case 0:
url = "";
break;
case 1:
url = "/";
break;
}
}
Modifier and Type | Method and Description |
---|---|
static int |
getExponent(double d) 返回a的表示中使用的無偏指數 |
static int |
getExponent(float f) 返回a的表示中使用的無偏指數 |
static double |
signum(double d) 返回引數的signum函式; 如果引數為零,則為零,如果引數大於零則為1.0,如果引數小於零,則為-1.0。 |
static float |
signum(float f) 返回引數的signum函式; 如果引數為零,則為零,如果引數大於零則為1.0f,如果引數小於零,則為-1.0f。 |
static double |
ulp(double d) 返回引數的ulp的大小。 |
static float |
ulp(float f) 返回引數的ulp的大小。 |
java原始碼
public final class Math {
private Math() {}
public static int getExponent(float f) {
/*
* Bitwise convert f to integer, mask out exponent bits, shift
* to the right and then subtract out float's bias adjust to
* get true exponent value
*/
return ((Float.floatToRawIntBits(f) & FloatConsts.EXP_BIT_MASK) >>
(FloatConsts.SIGNIFICAND_WIDTH - 1)) - FloatConsts.EXP_BIAS;
}
public static int getExponent(double d) {
/*
* Bitwise convert d to long, mask out exponent bits, shift
* to the right and then subtract out double's bias adjust to
* get true exponent value.
*/
return (int)(((Double.doubleToRawLongBits(d) & DoubleConsts.EXP_BIT_MASK) >>
(DoubleConsts.SIGNIFICAND_WIDTH - 1)) - DoubleConsts.EXP_BIAS);
}
public static double ulp(double d) {
int exp = getExponent(d);
switch(exp) {
case DoubleConsts.MAX_EXPONENT+1: // NaN or infinity
return Math.abs(d);
case DoubleConsts.MIN_EXPONENT-1: // zero or subnormal
return Double.MIN_VALUE;
default:
assert exp <= DoubleConsts.MAX_EXPONENT && exp >= DoubleConsts.MIN_EXPONENT;
// ulp(x) is usually 2^(SIGNIFICAND_WIDTH-1)*(2^ilogb(x))
exp = exp - (DoubleConsts.SIGNIFICAND_WIDTH-1);
if (exp >= DoubleConsts.MIN_EXPONENT) {
return powerOfTwoD(exp);
}
else {
// return a subnormal result; left shift integer
// representation of Double.MIN_VALUE appropriate
// number of positions
return Double.longBitsToDouble(1L <<
(exp - (DoubleConsts.MIN_EXPONENT - (DoubleConsts.SIGNIFICAND_WIDTH-1)) ));
}
}
}
public static float ulp(float f) {
int exp = getExponent(f);
switch(exp) {
case FloatConsts.MAX_EXPONENT+1: // NaN or infinity
return Math.abs(f);
case FloatConsts.MIN_EXPONENT-1: // zero or subnormal
return FloatConsts.MIN_VALUE;
default:
assert exp <= FloatConsts.MAX_EXPONENT && exp >= FloatConsts.MIN_EXPONENT;
// ulp(x) is usually 2^(SIGNIFICAND_WIDTH-1)*(2^ilogb(x))
exp = exp - (FloatConsts.SIGNIFICAND_WIDTH-1);
if (exp >= FloatConsts.MIN_EXPONENT) {
return powerOfTwoF(exp);
}
else {
// return a subnormal result; left shift integer
// representation of FloatConsts.MIN_VALUE appropriate
// number of positions
return Float.intBitsToFloat(1 <<
(exp - (FloatConsts.MIN_EXPONENT - (FloatConsts.SIGNIFICAND_WIDTH-1)) ));
}
}
}
public static double signum(double d) {
return (d == 0.0 || Double.isNaN(d))?d:copySign(1.0, d);
}
public static float signum(float f) {
return (f == 0.0f || Float.isNaN(f))?f:copySign(1.0f, f);
}
}
public final class StrictMath {
private StrictMath() {}
public static double ulp(double d) {
return Math.ulp(d);
}
public static float ulp(float f) {
return Math.ulp(f);
}
public static double signum(double d) {
return Math.signum(d);
}
public static float signum(float f) {
return Math.signum(f);
}
public static int getExponent(float f) {
return Math.getExponent(f);
}
public static int getExponent(double d) {
return Math.getExponent(d);
}
}