String str = "abc/";

StringOne.java
String str1 = "abc" + '/';
String str2 = "abc" + "/";
System.out.println(str1 == str2);

javap -v -l StringOne.class > StringOne.s

StringOne.s
#2 = String       #20      // abc/
...
#20 = Utf8        abc/
...
0: ldc      #2         // String abc/
2: astore_1
3: ldc      #2         // String abc/
5: astore_2

StringTwo
public static String stringAddString(String str1, String str2){
  return str1 + str2;
}

public static String stringAddChar(String str, char ch){
  return str + ch;
}

StringTwo.s
public java.lang.String stringAddString(java.lang.String, java.lang.String);
 descriptor: (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
 flags: ACC_PUBLIC
 Code:
  stack=2, locals=3, args_size=3
    0: new      #2         // class java/lang/StringBuilder
    3: dup
    4: invokespecial #3         // Method java/lang/StringBuilder."< init>":()V
    7: aload_1
    8: invokevirtual #4         // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   11: aload_2
   12: invokevirtual #4         // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   15: invokevirtual #5         // Method java/lang/StringBuilder. toString:()Ljava/lang/String;
   18: areturn

public java.lang.String stringAddChar(java.lang.String, char);
 descriptor: (Ljava/lang/String;C)Ljava/lang/String;
 flags: ACC_PUBLIC
 Code:
  stack=2, locals=3, args_size=3
    0: new      #2         // class java/lang/StringBuilder
    3: dup
    4: invokespecial #3         // Method java/lang/StringBuilder."<init>":()V
    7: aload_1
    8: invokevirtual #4         // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   11: iload_2
   12: invokevirtual #6         // Method java/lang/StringBuilder.append:(C)Ljava/lang/StringBuilder;
   15: invokevirtual #5         // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   18: areturn

public AbstractStringBuilder append(char c) {
  ensureCapacityInternal(count + 1);  // 确保数组能够容纳count+1个字符
  value[count++] = c;
  return this;
}

public AbstractStringBuilder append(String str) {
  if (str == null)
    return appendNull();
  int len = str.length();
  ensureCapacityInternal(count + len);
  str.getChars(0, len, value, count); // 拷贝字符串中的字符数组到本对象的字符数组中
  count += len;
  return this;
}

String str1 = "abc" + "123";  // 1

StringBuilder stringBuilder = new StringBuilder(); // 2
stringBuilder.append("abc");
stringBuilder.append("123");
String str2 = stringBuilder.toString();