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Java concatenate strings vs static strings

I try to get a better understanding of Strings. I am basically making a program that requires a lot of strings. However, a lot of the strings are very, very similar and merely require a different word at the end of the string.
E.g.
String one = "I went to the store and bought milk"
String two = "I went to the store and bought eggs"
String three = "I went to the store and bought cheese"
So my question is, what approach would be best suited to take when dealing with strings? Would concatenating 2 strings together have any benefits over just having static strings in, say for example, performance or memory management?
E.g.
String one = "I went to the store and bought "
String two = "milk"
String three = "cheese"
String four = one + two
String five = one + three
I am just trying to figure out the most optimal way of dealing with all these strings. (If it helps to put a number of strings I am using, I currently have 50 but the number could surplus a huge amount)

As spooky has said the main concern with the code is readability. Unless you are working on a program for a phone you do not need to manage your resources. That being said, it really doesn't matter whether you create a lot of Strings that stand alone or concatenate a base String with the small piece that varies. You won't really notice better performance either way.

You may set the opening sentence in a string like this
String openingSentence = "I went to the store and bought";
and alternate defining each word alone, by defining one array of strings like the following ::
String[] thingsToBeBought = { "milk", "water", "cheese" .... };
then you can do foreach loop and concatenate each element in the array with the opening sentence.

In Java, if you concatenate two Strings (e.g. using '+') a new String is created, so the old memory needs to be garbage collected. If you want to concatenate strings, the correct way to do this is to use a StringBuilder or StringBuffer.
Given your comment about these strings really being URLs, you probably want to have a StringBuilder/StringBuffer that is the URL base, and then append the suffixes as needed.

Performance wise final static strings are always better as they are generated during compile time. Something like this
final static String s = "static string";
Non static strings and strings concatenated as shown in the other example are generated at runtime. So even though performance will hardly matter for such a small thing, The second example is not as good as the first one performance wise as in your code :
// not as good performance wise since they are generated at runtime
String four = one + two
String five = one + three

Since you are going to use this string as URL, I would recommend to use StringJoiner (in case your are using JAVA 8). It will be as efficient as StringBuilder (will not create a new string every time you perform concatenation) and will automatically add "/" between strings.
StringJoiner myJoiner = new StringJoiner("/")

There will be no discernable difference in performance, so the manner in which you go about this is more a matter of preference. I would likely declare the first part of the sentence as a String and store the individual purchase items in an array.
Example:
String action = "I went to the store and bought ";
String [] items = {"milk", "eggs", "cheese"};
for (int x = 0; x< items.length; x++){
System.out.println(action + items[x]);
}

Whether you declare every possible String or separate Strings to be concatenated isn't going to have any measurable impact on memory or performance in the example you give. In the extreme case of declaring truly large numbers of String literals, Java's native hash table of interned Strings will use more memory if you declare every possible String, because the table's cached values will be longer.
If you are concatenating more than 2 Strings using the + operator, you will be creating extra String objects to be GC'd. For example if you have Strings a = "1" and b = "2", and do String s = "s" + a + b;, Java will first create the String "s1" and then concatenate it to form a second String "s12". Avoid the intermediate String by using something like StringBuilder. (This wouldn't apply to compile-time declarations, but it would to runtime concatenations.)
If you happen to be formatting a String rather than simply concatenating, use a MessageFormat or String.format(). It's prettier and avoids the intermediate Strings created when using the + operator. So something like, String urlBase = "http://host/res?a=%s&b=%s"; String url = String.format(urlBase, a, b); where a and b are the query parameter String values.

Is String Builder or ""+someNumb more efficient/correct for small cases?

II've been in the habit of doing:
int num = 12;
String text = ""+12;
for a long time, but I've found that to be a very inefficient mechanism for the large number of additions.
For those cases I generally do something like:
// this is psuedo code here..
FileInputStream fis = new FileInputStream(fis);
StringBuilder builder = new StringBuilder();
while(input.hasNext()) {
builder.append(input.nextString());
}
My question is: When coding for Android (vs the General Java case) Is the performance trade off at the small case worth using String Builder, or are there any other reasons to prefer String Builder in these small cases? It seems like it's a lot of extra typing int he simple case presented above. I also suspect (though I have not confirmed) that the memory allocations in the simple case are probably not worth it.
Edit: Suppose that the values being appended aren't known at compile time, I.E. they aren't constants.
Also the example above of ""+12 is a poorly chosen example.. Suppose it was
String userGeneratedText = textInput.getText().toString();
int someVal = intInput.getInt();
String finalVal = userGeneratedText+someVal;

If your code is short as you shown here:
String text = "foo" + 12;
The compiler will automatically replace the concatenation to use StringBuilder:
String text = new StringBuilder().append("foo").append(12).toString();
So don't worry about the inefficiency of this code, because it will work better than you expect.
For cases when you need to append very large Strings or you don't know how many objects (Strings, ints, booleans, etc) will you concatenate, use a StringBuilder as you do in your second code sample.
Here's a more in depth explanation about how the String concatenation works: http://blog.eyallupu.com/2010/09/under-hood-of-java-strings.html

As far as I know! string is immutable object. It means that its state cannot be changed, when ever you append value to string type then what happened is compiler deletes old one create new one with apended value.
But this is not the case with StringBuilder. StringBuilder is mutable which means its old value won't be destroyed. Any change/append will be taken place with existing object.
I know I am not covering in depth but this might cause major performance difference.

Java concatenate to build string or format

I'm writing a MUD (text based game) at the moment using java. One of the major aspects of a MUD is formatting strings and sending it back to the user. How would this best be accomplished?
Say I wanted to send the following string:
You say to Someone "Hello!" - where "Someone", "say" and "Hello!" are all variables. Which would be best performance wise?
"You " + verb + " to " + user + " \"" + text + "\""
or
String.format("You %1$s to %2$s \"%3$s\"", verb, user, text)
or some other option?
I'm not sure which is going to be easier to use in the end (which is important because it'll be everywhere), but I'm thinking about it at this point because concatenating with +'s is getting a bit confusing with some of the bigger lines. I feel that using StringBuilder in this case will simply make it even less readable.
Any suggestion here?

If the strings are built using a single concatenation expression; e.g.
String s = "You " + verb + " to " + user + " \"" + text + "\"";
then this is more or less equivalent to the more long winded:
StringBuilder sb = new StringBuilder();
sb.append("You");
sb.append(verb);
sb.append(" to ");
sb.append(user);
sb.append(" \"");
sb.append(text );
sb.append('"');
String s = sb.toString();
In fact, a classic Java compiler will compile the former into the latter ... almost. In Java 9, they implemented JEP 280 which replaces the sequence of constructor and method calls in the bytecodes with a single invokedynamic bytecode. The runtime system then optimizes this1.
The efficiency issues arise when you start creating intermediate strings, or building strings using += and so on. At that point, StringBuilder becomes more efficient because you reduce the number of intermediate strings that get created and then thrown away.
Now when you use String.format(), it should be using a StringBuilder under the hood. However, format also has to parse the format String each time you make the call, and that is an overhead you don't have if you do the string building optimally.
Having said this, My Advice would be to write the code in the way that is most readable. Only worry about the most efficient way to build strings if profiling tells you that this is a real performance concern. (Right now, you are spending time thinking about ways to address a performance issue that may turn out to be insignificant or irrelevant.)
Another answer mentions that using a format string may simplify support for multiple languages. This is true, though there are limits as to what you can do with respect to such things as plurals, genders, and so on.
1 - As a consequence, hand optimization as per the example above might actually have negative consequences, for Java 9 or later. But this is a risk you take whenever you micro-optimize.

I think that concatenation with + is more readable than using String.format.
String.format is good when you need to format number and dates.

Concateneting with plus, the compilet can transforms the code in performatic way. With string format i don t know.
I prefer cocatenation with plus, i think that is easer to undersand.

The key to keeping it simple is to never look at it. Here is what I mean:
Joiner join = Joiner.on(" ");
public void constructMessage(StringBuilder sb, Iterable<String> words) {
join.appendTo(sb, words);
}
I'm using the Guava Joiner class to make readability a non-issue. What could be clearer than "join"? All the nasty bits regarding concatenation are nicely hidden away. By using Iterable, I can use this method with all sorts of data structures, Lists being the most obvious.
Here is an example of a call using a Guava ImmutableList (which is more efficient than a regular list, since any methods that modify the list just throw exceptions, and correctly represents the fact that constructMessage() cannot change the list of words, just consume it):
StringBuilder outputMessage = new StringBuilder();
constructMessage(outputMessage,
new ImmutableList.Builder<String>()
.add("You", verb, "to", user, "\"", text, "\"")
.build());

I will be honest and suggest that you take the first one if you want less typing, or the latter one if you are looking for a more C-style way of doing it.
I sat here for a minute or two pondering the idea of what could be a problem, but I think it comes down to how much you want to type.
Anyone else have an idea?

Assuming you are going to reuse base strings often Store your templates like
String mystring = "You $1 to $2 \"$3\""
Then just get a copy and do a replace $X with what you want.
This would work really well for a resource file too.

I think String.format looks cleaner.
However you can use StringBuilder and use append function to create the string you want

The best, performance-wise, would probably be to use a StringBuffer.

append or + operator in StringBuffer?

In my project there are some code snippets which uses StringBuffer objects, and the small part of it is as follows
StringBuffer str = new StringBuffer();
str.append("new " + "String()");
so i was confused with the use of append method and the + operator.
ie the following code could be written as
str.append("new ").append("String()");
So are the two lines above same?(functionally yes but) Or is there any particular usage of them? ie performance or readability or ???
thanks.

In that case it's more efficient to use the first form - because the compiler will convert it to:
StringBuffer str = new StringBuffer();
str.append("new String()");
because it concatenates constants.
A few more general points though:
If either of those expressions wasn't a constant, you'd be better off (performance-wise) with the two calls to append, to avoid creating an intermediate string for no reason
If you're using a recent version of Java, StringBuilder is generally preferred
If you're immediately going to append a string (and you know what it is at construction time), you can pass it to the constructor

Actually the bytecode compiler will replace all string concatenation which involve non constants in a Java program with invocations of StringBuffer. That is
int userCount = 2;
System.out.println("You are the " + userCount + " user");
will be rewritten as
int userCount = 2;
System.out.println(new StringBuffer().append("You are the ").append(userCount).append(" user").toString());
That is at least what is observable when decompiling java class files compiled with JDK 5 or 6. See this post.

The second form is most efficient in terms of performance because there is only one string object that is created and is appended to the stringbuffer.
The first form creates three string objects 1) for "new" 2)for "new String" 3) for the concatenated result of 1) and 2). and this third string object is concatenated to the string buffer.

Unless you are working with concurrent systems, use StringBuilder instead of StringBuffer. Its faster but not thread-safe :)
It also shares the same API so its more or less a straight find/replace-

Is conversion to String using ("" + <int value>) bad practice?

Is conversion to String in Java using
"" + <int value>
bad practice? Does it have any drawbacks compared to String.valueOf(...)?
Code example:
int i = 25;
return "" + i;
vs:
int i = 25;
return String.valueOf(i);
Update: (from comment)
And what about Integer.toString(int i) compared to String.valueOf(...)?

I would always prefer the String.valueOf version: mostly because it shows what you're trying to do. The aim isn't string concatenation - it's conversion to a string, "the string value of i".
The first form may also be inefficient - depending on whether the compiler spots what you're doing. If it doesn't, it may be creating a new StringBuffer or StringBuilder and appending the value, then converting it to a string.
Funnily enough, I have an article about this very topic - written years and years ago; one of the first Java articles on my web site, IIRC.

There is also Integer.toString(int i), which gives you the option of getting the string as a hex value as well (by passing a second param of 16).
Edit I just checked the source of String class:
public static String valueOf(int i) {
return Integer.toString(i, 10);
}
And Integer class:
public static String toString(int i, int radix) {
if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
radix = 10;
/* Use the faster version */
if (radix == 10) {
return toString(i);
}
...
If you call String.valueOf(i), it calls Integer.toString(i, 10), which then calls Integer.toString(i).
So Integer.toString(i) should be very slighty faster than String.valueOf(i), since you'd be cutting out two function calls. (Although the first function call could be optimized away by the compiler.)
Of course, a readability argument could still be made for String.valueOf(), since it allows you to change the type of the argument (and even handles nulls!), and the performance difference is negligible.

Definitely use String.valueOf(i).
Although I'm not sure of the optimizations on the compiler side, worst case scenario if you use "" + :
"" creates a new empty string.
"" + creates a StringBuilder (Java 1.5-16)
"" is appended to the StringBuilder, then
In other words, there is a lot of overhead that occurs if you use string addition. This is why it is not recommended to use the + operator on strings in loops. In general, always use Boolean.valueOf, Integer.valueOf, String.valueOf... etc, when possible. You'll save both on memory and on overhead.

Regardless of any performance considerations I think the first variant is really ugly. IMHO it's a shame that this kind of "dynamic casting" is even possible in Java.

Yes, it is IMHO a bad practice.
It would require to memory allocations (unless compiler and/or JIT optimize them). What's more, it will make less evident, what this code tries to do.

Personally I dislike the style of "" + i, but that is really a preference/coding standards thing. Ideally the compiler would optimize those into equivalent code (although you would have to decompile to see if it actually does), but technically, without optimization, "" + i is more inefficient because it creates a StringBuilder object that wasn't needed.

Right off the bat all I can think of is that in the your first example more String objects will be created than in the second example (and an additional StringBuilder to actually perform the concatenation).
But what you are actualy trying to do is create a String object from a int not concatenate a String with an int, so go for the:
String.valueOf(...);
option,
So yes your first option is bad practice!

I wonder what is best for static final variables contributing to compile-time constants:
public static final int VIEW_TYPE_LABEL_FIELD = 1;
public static final int VIEW_TYPE_HEADER_FIELD = ;
...
List <String[]> listViewInfo = new ArrayList<>();
listViewInfo.add(new String[]{"Label/Field view", String.valueOf(VIEW_TYPE_LABEL_FIELD)});
listViewInfo.add(new String[]{"Header/Field view", "" + VIEW_TYPE_LABEL_FIELD});
The compiler can potentially replace the String expressions with a constant. Is one or the other more recognizable as a compile-time constant? Maybe easier for the ("" + ..) construct?