Replace String With Another in Java

How to replace part of a String in Java?

Use regex like this :

public static void main(String[] args) {
    String s = "Hello my Dg6Us9k. I am alive";
    String newString=s.replaceFirst("\\smy\\s\\w{7}", "");
    System.out.println(newString);
}

O/P :
Hello. I am alive

Replace a char in a string with another string?

For your specific problem( replacing the first 'a'):

public String replaceFirst(String regex,
String replacement)

Replaces the first substring of this string that matches the given
regular expression with the given replacement.

That is:

String s="aaaaa";
String res=s.replaceFirst("a","bbbb");

For a general solution:

public StringBuilder replace(int start,
int end,
String str)

Parameters:
start - The beginning index, inclusive.

end - The ending index, exclusive.

str - String that will replace previous contents.

That is:

String s="aaaaa";
StringBuilder sb=new StringBuilder(s);
String res=sb.replace(0,1,"bbbb").toString();

Replace part of a string between indexes in Java

Firstly, you cannot do it¹, since a String is immutable in java.

However, you can create a new String object with the desired value, using the String#substring() method (for example):

String s = "123456789";
String newString = s.substring(0, 3) + "foobar" + s.substring(3+3);
System.out.println(newString);

If you do want to achieve it efficiently, you could avoid creating some intermediate strings used by the concatinating and substring() method.

String s = "123456789";
StringBuilder sb = new StringBuilder();
char[] buff = s.toCharArray();
sb.append(buff , 0, 3).append("foobar");
sb.append(buff,3+3,buff.length -(3+3));
System.out.println(sb.toString());

However, if it is not done in a very tight loop - you should probably ignore it, and stick with the first and more readable solution.

(1) not easily anyway, it can be done with reflection - but it should be avoided.

replace String with another in java using regular Expression

You may use a constrained-width lookbehind solution like

public static String getMaskedValue(String value) {
    return value.replaceAll("(?<=^.{0,6}).", "*");
}

See the Java demo online.

The (?<=^.{0,6}). pattern matches any char (but a line break char, with .) that is preceded with 0 to 6 chars at the start of the string.

A note on the use of lookbehinds in Java regexps:

✽ Java accepts quantifiers within lookbehind, as long as the length of
the matching strings falls within a pre-determined range. For
instance, (?<=cats?) is valid because it can only match strings of
three or four characters. Likewise, (?<=A{1,10}) is valid.

how to replace ' \' with '/' in a java string

replaceAll() needs Strings as parameters.
So, if you write

path = path.replaceAll('\', '/');

it fails because you should have written

path = path.replaceAll("\", "/");

But this also fails because character '\' should be typed '\\'.

path = path.replaceAll("\\", "/");

And this will fail during execution giving you a PatternSyntaxException, because the fisr String is a regular expression (Thanks @Bhavik Shah for pointing it out). So, writing it as a RegEx, as @jlordo gave in his answer:

path = path.replaceAll("\\\\", "/");

Is what you were looking for.

To make optimal your core, you should make it independent of the Operating System, so use @Thai Tran's tip:

path = path.replaceAll("\\\\", File.separator);

But this fails throwing an StringIndexOutOfBoundsException (I don't know why). It works if you use replace() with no regular expressions:

path = path.replace("\\", File.separator);

Android - how to replace part of a string by another string?

It is working, but it wont modify the caller object, but returning a new String.

So you just need to assign it to a new String variable, or to itself:

string = string.replace("to", "xyz");

String newString = string.replace("to", "xyz");

API Docs

public String replace (CharSequence target, CharSequence replacement)

Since: API Level 1

Copies this string replacing
occurrences of the specified target
sequence with another sequence. The
string is processed from the beginning
to the end.

Parameters

target the sequence to replace.
replacement the replacement
sequence.

Returns the resulting string.

Throws NullPointerException if target or replacement is null.

Replace string values from any Object class - Java

Modify the value according to your requirements (See https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/String.html#replaceAll(java.lang.String,java.lang.String))

And then set the changed value onto the object.

        if (value instanceof java.lang.String) {
            String newValue = ((String)value).replace(regex, replacement);
            field.set(obj, newValue);

        }

Your case may be different, but in many cases if you're having to use reflection, you may be missing a simpler way to do things.