如果字符串出现在变量之后，为什么Java编译器不将它们串联起来呢？

Question

public class StringLiteralTest {
    static void testPrefix() {
        int i = 0;
        String prefixConcat = "a" + "b" + i;
    }

    static void testSuffix() {
        int i = 0;
        String suffixConcat = i + "c" + "d";
    }
}

Compiled from "StringLiteralTest.java"
public class StringLiteralTest {
  public StringLiteralTest();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  static void testPrefix();
    Code:
       0: iconst_0
       1: istore_0
       2: new           #15                 // class java/lang/StringBuilder
       5: dup
       6: ldc           #17                 // String ab
       8: invokespecial #19                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
      11: iload_0
      12: invokevirtual #22                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      15: invokevirtual #26                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      18: astore_1
      19: return

  static void testSuffix();
    Code:
       0: iconst_0
       1: istore_0
       2: new           #15                 // class java/lang/StringBuilder
       5: dup
       6: iload_0
       7: invokestatic  #35                 // Method java/lang/String.valueOf:(I)Ljava/lang/String;
      10: invokespecial #19                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
      13: ldc           #41                 // String c
      15: invokevirtual #43                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: ldc           #46                 // String d
      20: invokevirtual #43                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      23: invokevirtual #26                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      26: astore_1
      27: return
}

为什么编译器不能组合第二种方法中的字符串文字？

源文件是用默认的Oracle JDK8 javac编译的。

Answer 1

因为字符串级联运算符(+)在语法上是左关联的：

例如，表达式：

a + b + c

总是被视为意思：

(a + b) + c

诚然，如果b和c是字符串，则表达式等价于a+(b+c)。因此，编译器可以在特定情况下执行您建议的操作，但这并不是规范所要求的...