LineBreakMeasurer (Java SE 21 & JDK 21)
Description: declaration: module: java.desktop, package: java.awt.font, class: LineBreakMeasurer
Keywords: java.awt.font.LineBreakMeasurer class

Module java.desktop
Package java.awt.font

# Class LineBreakMeasurer

java.lang.Object
java.awt.font.LineBreakMeasurer

public final class LineBreakMeasurer extends Object
The `LineBreakMeasurer` class allows styled text to be broken into lines (or segments) that fit within a particular visual advance. This is useful for clients who wish to display a paragraph of text that fits within a specific width, called the wrapping width.

`LineBreakMeasurer` is constructed with an iterator over styled text. The iterator's range should be a single paragraph in the text. `LineBreakMeasurer` maintains a position in the text for the start of the next text segment. Initially, this position is the start of text. Paragraphs are assigned an overall direction (either left-to-right or right-to-left) according to the bidirectional formatting rules. All segments obtained from a paragraph have the same direction as the paragraph.

Segments of text are obtained by calling the method `nextLayout`, which returns a `TextLayout` representing the text that fits within the wrapping width. The `nextLayout` method moves the current position to the end of the layout returned from `nextLayout`.

`LineBreakMeasurer` implements the most commonly used line-breaking policy: Every word that fits within the wrapping width is placed on the line. If the first word does not fit, then all of the characters that fit within the wrapping width are placed on the line. At least one character is placed on each line.

The `TextLayout` instances returned by `LineBreakMeasurer` treat tabs like 0-width spaces. Clients who wish to obtain tab-delimited segments for positioning should use the overload of `nextLayout` which takes a limiting offset in the text. The limiting offset should be the first character after the tab. The `TextLayout` objects returned from this method end at the limit provided (or before, if the text between the current position and the limit won't fit entirely within the wrapping width).

Clients who are laying out tab-delimited text need a slightly different line-breaking policy after the first segment has been placed on a line. Instead of fitting partial words in the remaining space, they should place words which don't fit in the remaining space entirely on the next line. This change of policy can be requested in the overload of `nextLayout` which takes a `boolean` parameter. If this parameter is `true`, `nextLayout` returns `null` if the first word won't fit in the given space. See the tab sample below.

In general, if the text used to construct the `LineBreakMeasurer` changes, a new `LineBreakMeasurer` must be constructed to reflect the change. (The old `LineBreakMeasurer` continues to function properly, but it won't be aware of the text change.) Nevertheless, if the text change is the insertion or deletion of a single character, an existing `LineBreakMeasurer` can be 'updated' by calling `insertChar` or `deleteChar`. Updating an existing `LineBreakMeasurer` is much faster than creating a new one. Clients who modify text based on user typing should take advantage of these methods.

Examples:

Rendering a paragraph in a component

>      public void paint(Graphics graphics) {

>          float dx = 0f, dy = 5f;
>          Graphics2D g2d = (Graphics2D)graphics;
>          FontRenderContext frc = g2d.getFontRenderContext();

>          AttributedString text = new AttributedString(".....");
>          AttributedCharacterIterator paragraph = text.getIterator();

>          LineBreakMeasurer measurer = new LineBreakMeasurer(paragraph, frc);
>          measurer.setPosition(paragraph.getBeginIndex());
>          float wrappingWidth = (float)getSize().width;

>          while (measurer.getPosition() < paragraph.getEndIndex()) {

>              TextLayout layout = measurer.nextLayout(wrappingWidth);

>              dy += (layout.getAscent());
>              float dx = layout.isLeftToRight() ?
>                  0 : (wrappingWidth - layout.getAdvance());

>              layout.draw(graphics, dx, dy);
>              dy += layout.getDescent() + layout.getLeading();
>          }
>      }

Rendering text with tabs. For simplicity, the overall text direction is assumed to be left-to-right

>      public void paint(Graphics graphics) {

>          float leftMargin = 10, rightMargin = 310;
>          float[] tabStops = { 100, 250 };

>          // assume styledText is an AttributedCharacterIterator, and the number
>          // of tabs in styledText is tabCount

>          int[] tabLocations = new int[tabCount+1];

>          int i = 0;
>          for (char c = styledText.first(); c != styledText.DONE; c = styledText.next()) {
>              if (c == '\t') {
>                  tabLocations[i++] = styledText.getIndex();
>              }
>          }
>          tabLocations[tabCount] = styledText.getEndIndex() - 1;

>          // Now tabLocations has an entry for every tab's offset in
>          // the text.  For convenience, the last entry is tabLocations
>          // is the offset of the last character in the text.

>          LineBreakMeasurer measurer = new LineBreakMeasurer(styledText);
>          int currentTab = 0;
>          float verticalPos = 20;

>          while (measurer.getPosition() < styledText.getEndIndex()) {

>              // Lay out and draw each line.  All segments on a line
>              // must be computed before any drawing can occur, since
>              // we must know the largest ascent on the line.
>              // TextLayouts are computed and stored in a Vector;
>              // their horizontal positions are stored in a parallel
>              // Vector.

>              // lineContainsText is true after first segment is drawn
>              boolean lineContainsText = false;
>              boolean lineComplete = false;
>              float maxAscent = 0, maxDescent = 0;
>              float horizontalPos = leftMargin;
>              Vector layouts = new Vector(1);
>              Vector penPositions = new Vector(1);

>              while (!lineComplete) {
>                  float wrappingWidth = rightMargin - horizontalPos;
>                  TextLayout layout =
>                          measurer.nextLayout(wrappingWidth,
>                                              tabLocations[currentTab]+1,
>                                              lineContainsText);

>                  // layout can be null if lineContainsText is true
>                  if (layout != null) {
>                      layouts.addElement(layout);
>                      penPositions.addElement(Float.valueOf(horizontalPos));
>                      horizontalPos += layout.getAdvance();
>                      maxAscent = Math.max(maxAscent, layout.getAscent());
>                      maxDescent = Math.max(maxDescent,
>                          layout.getDescent() + layout.getLeading());
>                  } else {
>                      lineComplete = true;
>                  }

>                  lineContainsText = true;

>                  if (measurer.getPosition() == tabLocations[currentTab]+1) {
>                      currentTab++;
>                  }

>                  if (measurer.getPosition() == styledText.getEndIndex())
>                      lineComplete = true;
>                  else if (horizontalPos >= tabStops[tabStops.length-1])
>                      lineComplete = true;

>                  if (!lineComplete) {
>                      // move to next tab stop
>                      int j;
>                      for (j=0; horizontalPos >= tabStops[j]; j++) {}
>                      horizontalPos = tabStops[j];
>                  }
>              }

>              verticalPos += maxAscent;

>              Enumeration layoutEnum = layouts.elements();
>              Enumeration positionEnum = penPositions.elements();

>              // now iterate through layouts and draw them
>              while (layoutEnum.hasMoreElements()) {
>                  TextLayout nextLayout = (TextLayout) layoutEnum.nextElement();
>                  Float nextPosition = (Float) positionEnum.nextElement();
>                  nextLayout.draw(graphics, nextPosition.floatValue(), verticalPos);
>              }

>              verticalPos += maxDescent;
>          }
>      }

See Also:
  - `TextLayout`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `LineBreakMeasurer(AttributedCharacterIterator text, FontRenderContext frc)`
  Constructs a `LineBreakMeasurer` for the specified text.
  `LineBreakMeasurer(AttributedCharacterIterator text, BreakIterator breakIter, FontRenderContext frc)`
  Constructs a `LineBreakMeasurer` for the specified text.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `deleteChar(AttributedCharacterIterator newParagraph, int deletePos)`
  Updates this `LineBreakMeasurer` after a single character is deleted from the text, and sets the current position to the beginning of the paragraph.
  `int`
  `getPosition()`
  Returns the current position of this `LineBreakMeasurer`.
  `void`
  `insertChar(AttributedCharacterIterator newParagraph, int insertPos)`
  Updates this `LineBreakMeasurer` after a single character is inserted into the text, and sets the current position to the beginning of the paragraph.
  `TextLayout`
  `nextLayout(float wrappingWidth)`
  Returns the next layout, and updates the current position.
  `TextLayout`
  `nextLayout(float wrappingWidth, int offsetLimit, boolean requireNextWord)`
  Returns the next layout, and updates the current position.
  `int`
  `nextOffset(float wrappingWidth)`
  Returns the position at the end of the next layout.
  `int`
  `nextOffset(float wrappingWidth, int offsetLimit, boolean requireNextWord)`
  Returns the position at the end of the next layout.
  `void`
  `setPosition(int newPosition)`
  Sets the current position of this `LineBreakMeasurer`.

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### LineBreakMeasurer

    public LineBreakMeasurer(AttributedCharacterIterator text, FontRenderContext frc)
    Constructs a `LineBreakMeasurer` for the specified text.
    Parameters:
      `text` - the text for which this `LineBreakMeasurer` produces `TextLayout` objects; the text must contain at least one character; if the text available through `iter` changes, further calls to this `LineBreakMeasurer` instance are undefined (except, in some cases, when `insertChar` or `deleteChar` are invoked afterward - see below)
      `frc` - contains information about a graphics device which is needed to measure the text correctly; text measurements can vary slightly depending on the device resolution, and attributes such as antialiasing; this parameter does not specify a translation between the `LineBreakMeasurer` and user space
    See Also:
      - `insertChar(java.text.AttributedCharacterIterator, int)`
      - `deleteChar(java.text.AttributedCharacterIterator, int)`

  - ### LineBreakMeasurer

    public LineBreakMeasurer(AttributedCharacterIterator text, BreakIterator breakIter, FontRenderContext frc)
    Constructs a `LineBreakMeasurer` for the specified text.
    Parameters:
      `text` - the text for which this `LineBreakMeasurer` produces `TextLayout` objects; the text must contain at least one character; if the text available through `iter` changes, further calls to this `LineBreakMeasurer` instance are undefined (except, in some cases, when `insertChar` or `deleteChar` are invoked afterward - see below)
      `breakIter` - the `BreakIterator` which defines line breaks
      `frc` - contains information about a graphics device which is needed to measure the text correctly; text measurements can vary slightly depending on the device resolution, and attributes such as antialiasing; this parameter does not specify a translation between the `LineBreakMeasurer` and user space
    Throws:
      `IllegalArgumentException` - if the text has less than one character
    See Also:
      - `insertChar(java.text.AttributedCharacterIterator, int)`
      - `deleteChar(java.text.AttributedCharacterIterator, int)`

- ## Method Details

  - ### nextOffset

    public int nextOffset(float wrappingWidth)
    Returns the position at the end of the next layout. Does NOT update the current position of this `LineBreakMeasurer`.
    Parameters:
      `wrappingWidth` - the maximum visible advance permitted for the text in the next layout
    Returns:
      an offset in the text representing the limit of the next `TextLayout`.

  - ### nextOffset

    public int nextOffset(float wrappingWidth, int offsetLimit, boolean requireNextWord)
    Returns the position at the end of the next layout. Does NOT update the current position of this `LineBreakMeasurer`.
    Parameters:
      `wrappingWidth` - the maximum visible advance permitted for the text in the next layout
      `offsetLimit` - the first character that can not be included in the next layout, even if the text after the limit would fit within the wrapping width; `offsetLimit` must be greater than the current position
      `requireNextWord` - if `true`, the current position that is returned if the entire next word does not fit within `wrappingWidth`; if `false`, the offset returned is at least one greater than the current position
    Returns:
      an offset in the text representing the limit of the next `TextLayout`

  - ### nextLayout

    public TextLayout nextLayout(float wrappingWidth)
    Returns the next layout, and updates the current position.
    Parameters:
      `wrappingWidth` - the maximum visible advance permitted for the text in the next layout
    Returns:
      a `TextLayout`, beginning at the current position, which represents the next line fitting within `wrappingWidth`

  - ### nextLayout

    public TextLayout nextLayout(float wrappingWidth, int offsetLimit, boolean requireNextWord)
    Returns the next layout, and updates the current position.
    Parameters:
      `wrappingWidth` - the maximum visible advance permitted for the text in the next layout
      `offsetLimit` - the first character that can not be included in the next layout, even if the text after the limit would fit within the wrapping width; `offsetLimit` must be greater than the current position
      `requireNextWord` - if `true`, and if the entire word at the current position does not fit within the wrapping width, `null` is returned. If `false`, a valid layout is returned that includes at least the character at the current position
    Returns:
      a `TextLayout`, beginning at the current position, that represents the next line fitting within `wrappingWidth`. If the current position is at the end of the text used by this `LineBreakMeasurer`, `null` is returned

  - ### getPosition

    public int getPosition()
    Returns the current position of this `LineBreakMeasurer`.
    Returns:
      the current position of this `LineBreakMeasurer`
    See Also:
      - `setPosition(int)`

  - ### setPosition

    public void setPosition(int newPosition)
    Sets the current position of this `LineBreakMeasurer`.
    Parameters:
      `newPosition` - the current position of this `LineBreakMeasurer`; the position should be within the text used to construct this `LineBreakMeasurer` (or in the text most recently passed to `insertChar` or `deleteChar`
    See Also:
      - `getPosition()`

  - ### insertChar

    public void insertChar(AttributedCharacterIterator newParagraph, int insertPos)
    Updates this `LineBreakMeasurer` after a single character is inserted into the text, and sets the current position to the beginning of the paragraph.
    Parameters:
      `newParagraph` - the text after the insertion
      `insertPos` - the position in the text at which the character is inserted
    Throws:
      `IndexOutOfBoundsException` - if `insertPos` is less than the start of `newParagraph` or greater than or equal to the end of `newParagraph`
      `NullPointerException` - if `newParagraph` is `null`
    See Also:
      - `deleteChar(java.text.AttributedCharacterIterator, int)`

  - ### deleteChar

    public void deleteChar(AttributedCharacterIterator newParagraph, int deletePos)
    Updates this `LineBreakMeasurer` after a single character is deleted from the text, and sets the current position to the beginning of the paragraph.
    Parameters:
      `newParagraph` - the text after the deletion
      `deletePos` - the position in the text at which the character is deleted
    Throws:
      `IndexOutOfBoundsException` - if `deletePos` is less than the start of `newParagraph` or greater than the end of `newParagraph`
      `NullPointerException` - if `newParagraph` is `null`
    See Also:
      - `insertChar(java.text.AttributedCharacterIterator, int)`