The other answers explain well why only 1 String is added to the String pool. But if you want to check and make some tests by yourself you can take a look on the bytecode to see the number of String created and added to the string pool. Ex:
Ex1:
public static void main(String[] args) {
String hi = "Tom" + "Brady" + "Goat";
}
ByteCode:
// access flags 0x9
public static main(String[]) : void
L0
LINENUMBER 6 L0
LDC "TomBradyGoat"
ASTORE 1
L1
LINENUMBER 7 L1
RETURN
L2
LOCALVARIABLE args String[] L0 L2 0
LOCALVARIABLE hi String L1 L2 1
MAXSTACK = 1
MAXLOCALS = 2
As you can see only 1 String is created
Ex2:
public static void main(String[] args) {
String str1 = "Tom";
String str2 = "Brady";
String str3 = "Goat";
String str = str1 + str2 + str3;
}
Bytecode:
// access flags 0x9
public static main(String[]) : void
L0
LINENUMBER 6 L0
LDC "Tom"
ASTORE 1
L1
LINENUMBER 7 L1
LDC "Brady"
ASTORE 2
L2
LINENUMBER 8 L2
LDC "Goat"
ASTORE 3
L3
LINENUMBER 9 L3
NEW StringBuilder
DUP
ALOAD 1: str1
INVOKESTATIC String.valueOf (Object) : String
INVOKESPECIAL StringBuilder.<init> (String) : void
ALOAD 2: str2
INVOKEVIRTUAL StringBuilder.append (String) : StringBuilder
ALOAD 3: str3
INVOKEVIRTUAL StringBuilder.append (String) : StringBuilder
INVOKEVIRTUAL StringBuilder.toString () : String
ASTORE 4
L4
LINENUMBER 10 L4
RETURN
L5
LOCALVARIABLE args String[] L0 L5 0
LOCALVARIABLE str1 String L1 L5 1
LOCALVARIABLE str2 String L2 L5 2
LOCALVARIABLE str3 String L3 L5 3
LOCALVARIABLE str String L4 L5 4
MAXSTACK = 3
MAXLOCALS = 5
As you can see 4 Strings are created