JDK8 BigDecimal API-创建BigDecimal源码浅析三

第三篇

先介绍以BigInteger为构造参数的构造器

     public BigDecimal(BigInteger val) {// 根据BigInteger创建BigDecimal对象

         scale = 0;// BigInteger为整数因此有效小数位数为0

         intVal = val;

         intCompact = compactValFor(val);

     }

     public BigDecimal(BigInteger unscaledVal, int scale) {// 这个与上一个差不多但是指定了有效小数位数,但是最终的BigDecimal的数值为unscaledVal*10^-scale次方

         // Negative scales are now allowed

         this.intVal = unscaledVal;

         this.intCompact = compactValFor(unscaledVal);

         this.scale = scale;

     }

     public BigDecimal(BigInteger val, MathContext mc) {// 该方法转发调用下面的构造器

         this(val,0,mc);

     }

     public BigDecimal(BigInteger unscaledVal, int scale, MathContext mc) {

         long compactVal = compactValFor(unscaledVal);

         int mcp = mc.precision;

         int prec = 0;

         if (mcp > 0) { // do rounding,根据MathContext中的有效位数进行舍去操作,具体解析见第一篇BigDecimal源码解析文章

             int mode = mc.roundingMode.oldMode;

             if (compactVal == INFLATED) {

                 prec = bigDigitLength(unscaledVal);

                 int drop = prec - mcp;

                 while (drop > 0) {

                     scale = checkScaleNonZero((long) scale - drop);

                     unscaledVal = divideAndRoundByTenPow(unscaledVal, drop, mode);

                     compactVal = compactValFor(unscaledVal);

                     if (compactVal != INFLATED) {

                         break;

                     }

                     prec = bigDigitLength(unscaledVal);

                     drop = prec - mcp;

                 }

             }

             if (compactVal != INFLATED) {

                 prec = longDigitLength(compactVal);

                 int drop = prec - mcp;     // drop can't be more than 18

                 while (drop > 0) {

                     scale = checkScaleNonZero((long) scale - drop);

                     compactVal = divideAndRound(compactVal, LONG_TEN_POWERS_TABLE[drop], mode);

                     prec = longDigitLength(compactVal);

                     drop = prec - mcp;

                 }

                 unscaledVal = null;

             }

         }

         this.intVal = unscaledVal;

         this.intCompact = compactVal;

         this.scale = scale;

         this.precision = prec;// 若MathContext中的有效位数小于等于0,则BigDecimal中的有效位数置为0

     }

接下来介绍以int类型为构造参数的构造器

     public BigDecimal(int val) {// 以int数值来创建BigDecimal对象,int类型为整数则有效小数位数为0

         this.intCompact = val;

         this.scale = 0;

         this.intVal = null;// 此时BigDecimal的数值在int类型的表数范围因此也在long类型的表数范围,所以intVal为null

     }

     public BigDecimal(int val, MathContext mc) {// 该构造器在以int类型为参数的同时传入一个MathContext来限制有效位数

         int mcp = mc.precision;

         long compactVal = val;

         int scale = 0;

         int prec = 0;

         if (mcp > 0) { // do rounding,根据val的位数与MathContext的有效位数修正最终值的有效位数,即进行舍去操作,具体分析见第一篇BigDecimal源码分析文章

             prec = longDigitLength(compactVal);

             int drop = prec - mcp; // drop can't be more than 18

             while (drop > 0) {

                 scale = checkScaleNonZero((long) scale - drop);

                 compactVal = divideAndRound(compactVal, LONG_TEN_POWERS_TABLE[drop], mc.roundingMode.oldMode);

                 prec = longDigitLength(compactVal);

                 drop = prec - mcp;

             }

         }

         this.intVal = null;

         this.intCompact = compactVal;// BigDecimal对象表示数值的简洁值就是int类型参数val

         this.scale = scale;

         this.precision = prec;

     }

long类型参数的构造器分析

     public BigDecimal(long val) {// 以long类型数值来创建BigDecimal对象,long类型为整数则有效小数位数为0

         this.intCompact = val;

         this.intVal = (val == INFLATED) ? INFLATED_BIGINT : null;// 若val的数值为long类型最小值需要特殊处理,因为此时的val有特殊含义(数值在long类型下溢出)

         this.scale = 0;

     }

     public BigDecimal(long val, MathContext mc) {// 该构造器在以long类型为参数的同时传入一个MathContext来限制有效位数

         int mcp = mc.precision;

         int mode = mc.roundingMode.oldMode;

         int prec = 0;

         int scale = 0;

         BigInteger intVal = (val == INFLATED) ? INFLATED_BIGINT : null;

         if (mcp > 0) { // do rounding,根据val的位数与MathContext的有效位数修正最终值的有效位数,即进行舍去操作,具体分析见第一篇BigDecimal源码分析文章

             if (val == INFLATED) {// 若val为INFLATED即-2^63,该数位数为19,因此初始化有效位数为19

                 prec = 19;

                 int drop = prec - mcp;

                 while (drop > 0) {

                     scale = checkScaleNonZero((long) scale - drop);

                     intVal = divideAndRoundByTenPow(intVal, drop, mode);

                     val = compactValFor(intVal);

                     if (val != INFLATED) {

                         break;

                     }

                     prec = bigDigitLength(intVal);

                     drop = prec - mcp;

                 }

             }

             if (val != INFLATED) {

                 prec = longDigitLength(val);

                 int drop = prec - mcp;

                 while (drop > 0) {

                     scale = checkScaleNonZero((long) scale - drop);

                     val = divideAndRound(val, LONG_TEN_POWERS_TABLE[drop], mc.roundingMode.oldMode);

                     prec = longDigitLength(val);

                     drop = prec - mcp;

                 }

                 intVal = null;

             }

         }

         this.intVal = intVal;

         this.intCompact = val;

         this.scale = scale;

         this.precision = prec;

     }

BigDecimal的原码接下来是一堆的静态方法用于创建BigDecimal对象,几乎没有什么需要分析的,很简单大家可以自己看一看