添加计算方法。

src/main/java/xyz/zhouxy/plusone/commons/time/YearQuarter.java

@@ -1,5 +1,7 @@
 package xyz.zhouxy.plusone.commons.time;
 
+import static java.time.temporal.ChronoField.YEAR;
+
 import java.io.Serializable;
 import java.time.LocalDate;
 import java.time.Month;
@@ -29,6 +31,14 @@ public final class YearQuarter implements Comparable<YearQuarter>, Serializable
     /** 季度结束日期 */
     private final LocalDate lastDate;
 
+    private YearQuarter(int year, int quarter) {
+        Preconditions.checkNotNull(quarter, "Quarter can not be null.");
+        this.year = year;
+        this.quarter = Quarter.of(quarter);
+        this.firstDate = this.quarter.firstMonthDay().atYear(year);
+        this.lastDate = this.quarter.lastMonthDay().atYear(year);
+    }
+
     private YearQuarter(int year, @Nonnull Quarter quarter) {
         Preconditions.checkNotNull(quarter, "Quarter can not be null.");
         this.year = year;
@@ -136,6 +146,37 @@ public final class YearQuarter implements Comparable<YearQuarter>, Serializable
 
     // Getters end
 
+    // computes
+
+    public YearQuarter plusQuarters(long quartersToAdd) { // TODO 单元测试
+        if (quartersToAdd == 0) {
+            return this;
+        }
+        long quarterCount = this.year * 4L + (this.quarter.getValue() - 1);
+        long calcQuarters = quarterCount + quartersToAdd; // safe overflow
+        int newYear = YEAR.checkValidIntValue(Math.floorDiv(calcQuarters, 4));
+        int newQuarter = (int) Math.floorMod(calcQuarters, 4) + 1;
+        return of(newYear, Quarter.of(newQuarter));
+    }
+
+    public YearQuarter minusQuarters(long quartersToAdd) { // TODO 单元测试
+        return plusQuarters(-quartersToAdd);
+    }
+
+    public YearQuarter plusYears(long yearsToAdd) { // TODO 单元测试
+        if (yearsToAdd == 0) {
+            return this;
+        }
+        int newYear = YEAR.checkValidIntValue(this.year + yearsToAdd);  // safe overflow
+        return of(newYear, this.quarter);
+    }
+
+    public YearQuarter minusYears(long yearsToAdd) {
+        return plusYears(-yearsToAdd);
+    }
+
+    // computes end
+
     // hashCode & equals
 
     @Override