类介绍
public class BigDecimal extends Number implements Comparable<BigDecimal> {// 该BigDecimal的未缩放值private final BigInteger intVal;// 精度，可以理解成小数点后的位数private final int scale;// BigDecimal中的十进制位数，如果位数未知，则为0(备用信息)private transient int precision;// Used to store the canonical string representation, if computed.// 这个我理解就是存实际的BigDecimal值private transient String stringCache;// 扩大成long型数值后的值private final transient long intCompact;}
从例子入手
@Testpublic void testBigDecimal() {BigDecimal bigDecimal1 = BigDecimal.valueOf(2.36);BigDecimal bigDecimal2 = BigDecimal.valueOf(3.5);BigDecimal resDecimal = bigDecimal1.add(bigDecimal2);System.out.println(resDecimal);}
/*** Returns a BigDecimal whose value is (this + augend),* and whose scale is max(this.scale(), augend.scale()).*/public BigDecimal add(BigDecimal augend) {if (this.intCompact != INFLATED) {if ((augend.intCompact != INFLATED)) {return add(this.intCompact, this.scale, augend.intCompact, augend.scale);} else {return add(this.intCompact, this.scale, augend.intVal, augend.scale);}} else {if ((augend.intCompact != INFLATED)) {return add(augend.intCompact, augend.scale, this.intVal, this.scale);} else {return add(this.intVal, this.scale, augend.intVal, augend.scale);}}}
private static BigDecimal add(final long xs, int scale1, final long ys, int scale2) {long sdiff = (long) scale1 - scale2;if (sdiff == 0) {return add(xs, ys, scale1);} else if (sdiff < 0) {int raise = checkScale(xs,-sdiff);long scaledX = longMultiplyPowerTen(xs, raise);if (scaledX != INFLATED) {return add(scaledX, ys, scale2);} else {BigInteger bigsum = bigMultiplyPowerTen(xs,raise).add(ys);return ((xs^ys)>=0) ? // same sign testnew BigDecimal(bigsum, INFLATED, scale2, 0): valueOf(bigsum, scale2, 0);}} else {int raise = checkScale(ys,sdiff);long scaledY = longMultiplyPowerTen(ys, raise);if (scaledY != INFLATED) {return add(xs, scaledY, scale1);} else {BigInteger bigsum = bigMultiplyPowerTen(ys,raise).add(xs);return ((xs^ys)>=0) ?new BigDecimal(bigsum, INFLATED, scale1, 0): valueOf(bigsum, scale1, 0);}}}
首先17行校验了一下数值范围 18行将ys扩大了10的n次倍，这里n=raise=1，所以返回的scaledY=350 接着就进入到20行的add方法：
private static BigDecimal add(long xs, long ys, int scale){long sum = add(xs, ys);if (sum!=INFLATED)return BigDecimal.valueOf(sum, scale);return new BigDecimal(BigInteger.valueOf(xs).add(ys), scale);}
结论
链接：https://juejin.cn/post/7348709938023940136
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