等额本息计算

等额本息计算

设总贷款金额为P,月利率为a,总期次为m
设每月还款为X元，则有
第一个月还款剩余本金 P(1+a)-X
第二个月还款剩余本金 (P(1+a)-X)(1+a)-X=P(1+a)^2-X(1+(1+a))
第三个月还款剩余本金 ((P(1+a)-X)(1+a)-X)(1+a)-X
=(P(1+a)^2-X(1+(1+a)))(1+a)-X = P(1+a)^3-X(1+(1+a)+(1+a)^2)
...
第m个月还款剩余本金 P(1+a)^m-X(1+(1+a)+(1+a)^2+...+(1+a)^(m-1))
= P(1+a)^m - X ( (1+a)^(m-1) / (1+a-1) = P(1+a)^m - X ( ( (1+a)^m - 1 ) / a )
因为第m个月刚好还完钱，所以
P(1+a)^m - X ( ( (1+a)^m - 1 ) / a ) = 0
=> P(1+a)^m = X ( ( (1+a)^m - 1 ) / a )
=> X = P ((1+a)^m ) a / ( (1+a)^m - 1 )

以下为代码实现：

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package com.jt;   import java.text.DecimalFormat; import com.alibaba.fastjson.JSONArray; import com.alibaba.fastjson.JSONObject; public class InterestCalc {       public static JSONArray AverageCapitalPlusInterest(Double principal, Double interest ,int period ) {         DecimalFormat dformat2 = new DecimalFormat("0.00");         JSONArray  jsarray = new JSONArray();                   Double monthpay =  principal  *  interest * Math.pow((1+interest) , period)  / ( Math.pow((1+interest) , period) - 1 ) ;         monthpay = Double.parseDouble( dformat2.format( monthpay) ); //锁定两位小数,精确到分           Double totalLeftPrincipal = principal;         Double totalInterest = 0.00;         for(int i=0; i<period; i++) {             JSONObject  jsOject = new JSONObject();             if(i<period-1) {                 jsOject.put("total", dformat2.format( monthpay) ); //每期还款金额                 Double tempInterest = totalLeftPrincipal * interest; //当期利息                 tempInterest = Double.parseDouble( dformat2.format(tempInterest) );                                   totalInterest = totalInterest + tempInterest;                 Double tempTotalLeftPrincipal = totalLeftPrincipal + tempInterest - monthpay ; //剩余本金                 Double currentPeriodPrincipal = monthpay - tempInterest ; //当期还款本金                 totalLeftPrincipal =  totalLeftPrincipal - currentPeriodPrincipal ; //剩余本金                 jsOject.put("principal", dformat2.format(currentPeriodPrincipal) );                 jsOject.put("interest", dformat2.format(tempInterest) );                 jsOject.put("leftPrincipal", dformat2.format(tempTotalLeftPrincipal) );             }else {                 Double tempInterest = totalLeftPrincipal * interest; //当期利息                 tempInterest = Double.parseDouble( dformat2.format(tempInterest) );                 totalInterest = totalInterest + tempInterest;                               Double total = Double.parseDouble(dformat2.format(tempInterest)) + Double.parseDouble(dformat2.format(totalLeftPrincipal));                 Double tempPrincipal = totalLeftPrincipal; //当期还款本金                 jsOject.put("total", total.toString() ); //最后一期剩余金额                 jsOject.put("principal", dformat2.format(tempPrincipal) );                 jsOject.put("interest", dformat2.format(tempInterest) );                 jsOject.put("leftPrincipal",0.00 ); //最后一期，剩余本金为0             }             jsarray.add(i, jsOject);         }         //总和         JSONObject  jsOject = new JSONObject();         jsOject.put("total", dformat2.format( principal + totalInterest ) ); //所有本金和利息的总金额         jsOject.put("principal", dformat2.format( principal ) ); //所有的本金         jsOject.put("interest", dformat2.format(totalInterest) );//所有的利息         jsOject.put("leftPrincipal",dformat2.format(100*totalInterest/principal) ); //         jsarray.add(period, jsOject);         return jsarray;     }           public static void main(String args[]) {         JSONArray  jsarrayResult = AverageCapitalPlusInterest(10000.00, 0.01, 12);         DecimalFormat dformat = new DecimalFormat("0.00");         for(int i=0; i<jsarrayResult.size(); i++) {             JSONObject tempObject = JSONArray.parseObject(jsarrayResult.get(i).toString());             System.out.print( dformat.format( tempObject.getDouble("total")  ) );             System.out.print( "-" );             System.out.print( dformat.format(  tempObject.getDouble("principal") ) );             System.out.print( "-" );             System.out.print( dformat.format(  tempObject.getDouble("interest") ) );             System.out.print( "-" );             System.out.println( dformat.format(  tempObject.getDouble("leftPrincipal") ) );         }     } }