The compareTo() method compares two strings lexicographically.

The comparison is based on the Unicode value of each character in the strings.

You need to call length() method for each string to compare lengths

if(one.length() > two.length()) ...

String::length is a method reference. Using it is equivalent to

Comparator.comparing(s -> s.length())

So it compares strings by comparing their length.

A simple solution is to iterate over all names and compare the length using the .length() method.

Another solution is to compare the length each time you enter a string (only one for loop). Here's an example :

int nb = 2;
String[] names = new String[nb];
int maxLength = -1; // Default value
for (int i = 0; i < nb; i++) {
    names[i] = JOptionPane.showInputDialog("Enter your name : ");
    // Compare the length of this name to the previous greatest length
    if (names[i].length() > maxLength) {
        // Change the maximum
        maxLength = names[i].length();
    }
}
JOptionPane.showMessageDialog(null, "The greatest value is " + maxLength);

You need to use:

test.equals("example")

Because you are checking if the objects are equal, not if the object values are equal.

This is a pretty common mistake, but thankfully it has a very simple fix. :)

== tests for reference equality (whether they are the same object).

.equals() tests for value equality (whether they contain the same data).

Objects.equals() checks for null before calling .equals() so you don't have to (available as of JDK7, also available in Guava).

Consequently, if you want to test whether two strings have the same value you will probably want to use Objects.equals().

// These two have the same value
new String("test").equals("test") // --> true 

// ... but they are not the same object
new String("test") == "test" // --> false 

// ... neither are these
new String("test") == new String("test") // --> false 

// ... but these are because literals are interned by 
// the compiler and thus refer to the same object
"test" == "test" // --> true 

// ... string literals are concatenated by the compiler
// and the results are interned.
"test" == "te" + "st" // --> true

// ... but you should really just call Objects.equals()
Objects.equals("test", new String("test")) // --> true
Objects.equals(null, "test") // --> false
Objects.equals(null, null) // --> true

From the Java Language Specification JLS 15.21.3. Reference Equality Operators == and !=:

While == may be used to compare references of type String, such an equality test determines whether or not the two operands refer to the same String object. The result is false if the operands are distinct String objects, even if they contain the same sequence of characters (§3.10.5, §3.10.6). The contents of two strings s and t can be tested for equality by the method invocation s.equals(t).

You almost always want to use Objects.equals(). In the rare situation where you know you're dealing with interned strings, you can use ==.

From JLS 3.10.5. String Literals:

Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances, using the method String.intern.

Similar examples can also be found in JLS 3.10.5-1.

Other Methods To Consider

String.equalsIgnoreCase() value equality that ignores case. Beware, however, that this method can have unexpected results in various locale-related cases, see this question.

String.contentEquals() compares the content of the String with the content of any CharSequence (available since Java 1.5). Saves you from having to turn your StringBuffer, etc into a String before doing the equality comparison, but leaves the null checking to you.